installation: Review recommended system specification for sysupdate installation

### Problem

Installation with sysupdate support (https://github.com/elementary/os/pull/809) failed with 32GB storage which we now require in [the recommended system specification](https://elementary.io/en/docs/installation#recommended-system-specifications).

```
Syncing future partition 0 contents to disk.
Adding new partition 0 to partition table.
Writing new partition table.
Informing kernel about changed partitions...
All done.
Mounting /dev/vda1 to /mnt/future-root as a scratch buffer...
Decompressing image into the scratch buffer...
Configuring partitions...
Can't fit requested partitions into available free space (11.9G), refusing.
Automatically determined minimal disk image size as 36.7G, current block device/image size is 32G.
```

Also now we require other things like UEFI with sysupdate image, so it's time to review and update the recommended system specification.

FYI I can confirm the installation succeeds with 64 GB:

Installation log with 64 GB storage

```
elementary@elementary:~$ run0 elementary-install
Thank you for testing! <3
Using source image: /cdrom/extra/elementary_20260912010023.raw.zst
1) /dev/zram0
2) /dev/vda
Select installation disk (ALL DATA WILL BE LOST): 2
/dev/vda selected for repartitioning...
Destroying drive in 3s ...
Destroying drive in 2s ...
Destroying drive in 1s ...
Creating root partition...
/opt/repart-target/root/40-root.conf:8: Missing '=', ignoring line.
/opt/repart-target/root/40-root.conf:9: Missing '=', ignoring line.
/opt/repart-target/root/40-root.conf:10: Missing '=', ignoring line.
TYPE LABEL UUID PART FILE RAW SIZE SIZE PADDING
root-x86-64 elementary-root 41d3d220-4fa3-42cc-93b4-cc5d4e4a4165 0 40-root.conf 21474836480 → 20G → 0B
Σ = 20G Σ = 0B

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
└─ 40-root.conf

Applying changes to /dev/vda.
Wiped block device.
Discarded entire block device.
Successfully wiped file system signatures from future partition 0.
Formatting future partition 0.
mke2fs 1.47.2 (1-Jan-2025)
Discarding device blocks: done
Creating filesystem with 5242880 4k blocks and 1310720 inodes
Filesystem UUID: 2faa2de9-ecc5-4286-806a-a8085c6a3538
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

/dev/loop1 successfully formatted as ext4 (label "elementary-root", uuid 2faa2de9-ecc5-4286-806a-a8085c6a3538)
Successfully formatted future partition 0.
Populating ext4 filesystem.
Successfully populated ext4 filesystem.
Syncing future partition 0 contents to disk.
Adding new partition 0 to partition table.
Writing new partition table.
Informing kernel about changed partitions...
All done.
Mounting /dev/vda1 to /mnt/future-root as a scratch buffer...
Decompressing image into the scratch buffer...
Configuring partitions...
TYPE LABEL UUID PART FILE RAW SIZE SIZE PADDING
esp elementary_20260912010023 dc48779d-b0ac-475a-aa49-3… 1 source.raw 1610612736 → 1.5G → 0B
usr-x86-64-verity-sig elementary_2026091201002… 4488839a-a8b4-4cbe-aa2d-2… 2 source.raw 16384 → 16K → 0B
usr-x86-64-verity elementary_2026091201002… d50481db-2e06-f9b1-8891-1… 3 source.raw 419430400 → 400M → 0B
usr-x86-64 elementary_20260912010023 69ad8853-60cc-03ed-85b6-8… 4 source.raw 7516192768 → 7G → 0B
usr-x86-64-verity-sig _empty 8427ad67-4651-49a0-af48-7… 5 source.raw 16384 → 16K → 0B
usr-x86-64-verity _empty cbfafb08-f901-46f4-8e70-5… 6 source.raw 419430400 → 400M → 0B
usr-x86-64 _empty e44c6708-c5b0-433f-a56c-0… 7 source.raw 7516192768 → 7G → 0B
home elementary-home e792f1d1-5cf5-4574-8106-5… 8 50-home.conf 29761679360 → 27.7G → 0B
root-x86-64 elementary-root 41d3d220-4fa3-42cc-93b4-c… 0 21474836480 20G 43.9G → 0B
Σ = 63.9G Σ = 0B

▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒░
└─ elementary-root │ │ │ │
│ │ │ └─ 50-home.conf
│ │ │
│ │ ├─ source.raw
│ │ └─ source.raw
│ │
│ ├─ source.raw
│ └─ source.raw
└─ source.raw

Applying changes to /dev/vda.
Successfully wiped file system signatures from future partition 1.
Successfully discarded data from future partition 1.
Successfully wiped file system signatures from future partition 2.
Successfully discarded data from future partition 2.
Successfully wiped file system signatures from future partition 3.
Successfully discarded data from future partition 3.
Successfully wiped file system signatures from future partition 4.
Successfully discarded data from future partition 4.
Successfully wiped file system signatures from future partition 5.
Successfully discarded data from future partition 5.
Successfully wiped file system signatures from future partition 6.
Successfully discarded data from future partition 6.
Successfully wiped file system signatures from future partition 7.
Successfully discarded data from future partition 7.
Successfully wiped file system signatures from future partition 8.
Successfully discarded data from future partition 8.
Successfully discarded gap after partition 8.
Copying in '/mnt/future-root/source.raw' @ 1048576 (1.5G) on block level into future partition 1.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 1 contents to disk.
Block level copying and synchronization of partition 1 complete in 1.773464s (866.1M/s).
Copying in '/mnt/future-root/source.raw' @ 1611661312 (16K) on block level into future partition 2.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 2 contents to disk.
Block level copying and synchronization of partition 2 complete in 6.031ms.
Copying in '/mnt/future-root/source.raw' @ 1611677696 (400M) on block level into future partition 3.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 3 contents to disk.
Block level copying and synchronization of partition 3 complete in 499.562ms (800.7M/s).
Copying in '/mnt/future-root/source.raw' @ 2031108096 (7G) on block level into future partition 4.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 4 contents to disk.
Block level copying and synchronization of partition 4 complete in 7.015305s (1021.7M/s).
Copying in '/mnt/future-root/source.raw' @ 9547300864 (16K) on block level into future partition 5.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 5 contents to disk.
Block level copying and synchronization of partition 5 complete in 4.594ms.
Copying in '/mnt/future-root/source.raw' @ 9547317248 (400M) on block level into future partition 6.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 6 contents to disk.
Block level copying and synchronization of partition 6 complete in 535.933ms (746.3M/s).
Copying in '/mnt/future-root/source.raw' @ 9966747648 (7G) on block level into future partition 7.
Copying in of '/mnt/future-root/source.raw' on block level completed.
Syncing future partition 7 contents to disk.
Block level copying and synchronization of partition 7 complete in 7.478763s (958.4M/s).
Formatting future partition 8.
mke2fs 1.47.2 (1-Jan-2025)
Creating filesystem with 7266035 4k blocks and 1818624 inodes
Filesystem UUID: a5e215b8-b607-4751-8a51-300ddb5dac53
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000

Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

/dev/loop2 successfully formatted as ext4 (label "elementary-home", uuid a5e215b8-b607-4751-8a51-300ddb5dac53)
Successfully formatted future partition 8.
Populating ext4 filesystem.
Successfully populated ext4 filesystem.
Syncing future partition 8 contents to disk.
Adding new partition 1 to partition table.
Adding new partition 2 to partition table.
Adding new partition 3 to partition table.
Adding new partition 4 to partition table.
Adding new partition 5 to partition table.
Adding new partition 6 to partition table.
Adding new partition 7 to partition table.
Adding new partition 8 to partition table.
Writing new partition table.
Informing kernel about changed partitions...
All done.
Cleaning up scratch buffer...
Everything is done! Enjoy your stay.
elementary@elementary:~$
```

### Proposal

Review recommended system specification in [installation.md](https://github.com/elementary/website/blob/master/docs/installation.md#recommended-system-specifications-recommended-system-specifications).

- Require (not recommend) UEFI with secure boot disabled
- `Solid state drive (SSD) with at least 32 GB of free space`: Should require 64 GB
- `You will also need a spare USB flash drive with at least 4 GB of storage for installation.`: Should require 8 GB; see the file size of the ISO image below

```
ryonakano@fedora-host:~/work/elementary/elementary-atomic-os (sysupdate =)$ LANG=C.UTF-8 ls -lh mkosi.output/elementaryos-9.0-daily-x86-64.20260912010023.iso
-rw-r--r--. 1 qemu qemu 5.3G Sep 12 10:03 mkosi.output/elementaryos-9.0-daily-x86-64.20260912010023.iso
ryonakano@fedora-host:~/work/elementary/elementary-atomic-os (sysupdate =)$
```

### Prior Art (Optional)

_No response_