Update `ssh_filter_btrbk.sh` to allow `stream_compress` with adaptive compression

Currently my `btrbk.conf` contains:
```
stream_compress_adapt yes
stream_compress zstd
```

I have my backup NAS set to run with `ssh_filter_btrbk.sh`. When I do it fails with something like:
```
ERROR: Failed to send/receive subvolume: /.btrbk_snap/home.20240903T0000 [//.btrbk_snap/home.20240902T0000] -> NAS:/mnt/main/btrbk/PC/home.20240903T0000
ERROR: ... SSH command failed (exitcode=255)
ERROR: ... sh: btrfs send -p '//.btrbk_snap/home.20240902T0000' --compressed-data '/.btrbk_snap/home.20240903T0000' | mbuffer -v 1 -q -m 256m | zstd -c --adapt | ssh -i '/root/.ssh/id_ed25519.btrbk' -o compression=no root@NAS 'zstd -d -c --adapt | btrfs receive '\''/mnt/main/btrbk/PC/'\'''
ERROR: ... ssh_filter_btrbk.sh: ssh command rejected: disallowed command (restrict-path: "/mnt/NAS/btrbk/PC"): zstd -d -c --adapt | btrfs receive '/mnt/main/btrbk/PC/'
ERROR: ... failed to check target subvolume: NAS:/mnt/main/btrbk/PC/home.20240903T0000
ERROR: ... Command execution failed (exitcode=1)
```

It would be nice if the default `ssh_filter_btrbk.sh` permitted this user of `zstd`.