Rebase on fd.o platform instead of GNOME one?

### Problem

The GNOME platform 49 has a regression that the cursor gets big in the app windows with Mutter we ship on OS 8.x:

https://github.com/elementary/flatpak-platform/pull/215#issuecomment-3559502427
https://github.com/elementary/gala/issues/2683

### Proposal

Give a shot if we should build our flatpak platform on the freedesktop platform directly instead of the GNOME one.

- [x] Create a test branch that based on fd.o runtime and uses newer vessels of GTK, libadwaita, etc. in the flatpak-platform repo (maybe version 9 because it’s a breaking change): #220
- [x] Build and install it locally
- [ ] Check if all flatpak manifests we maintain build succeed locally and the cursor issue is fixed after switching the based runtime to it
- [ ] browser
- [ ] calculator
- [ ] calendar
- [ ] camera
- [ ] capnet-assist
- [ ] code
- [ ] contacts
- [ ] evince
- [ ] fileroller
- [ ] font-viewer
- [ ] iconbrowser
- [ ] mail
- [ ] maps
- [ ] music
- [ ] papers
- [ ] photos
- [ ] screenshot
- [ ] tasks
- [ ] videos
- [ ] Merge the change into the flatpak-platform repo

### Prior Art (Optional)

_No response_