release-notes: document nixos-rebuild changes

- `fileSystems.<name>.autoResize` now uses `systemd-growfs` to resize the file system online in stage 2. This means that `f2fs` and `ext2` can no longer be auto resized, while `xfs` and `btrfs` now can be.

- `nixos-rebuild {switch,boot,test,dry-activate}` now runs the system activation inside `systemd-run`, creating an ephemeral systemd service and protecting the system switch against issues like network disconnections during remote (e.g. SSH) sessions. This has the side effect of running the switch in an isolated environment, that could possible break post-switch scripts that depends on things like environment variables being set. If you want to opt-out from this behavior for now, you may set the `NIXOS_SWITCH_USE_DIRTY_ENV` environment variable before running `nixos-rebuild`. However, keep in mind that this option will be removed in the future.

- The `services.vaultwarden.config` option default value was changed to make Vaultwarden only listen on localhost, following the [secure defaults for most NixOS services](https://github.com/NixOS/nixpkgs/issues/100192).

- `services.lemmy.settings.federation` was removed in 0.17.0 and no longer has any effect. To enable federation, the hostname must be set in the configuration file and then federation must be enabled in the admin web UI. See the [release notes](https://github.com/LemmyNet/lemmy/blob/c32585b03429f0f76d1e4ff738786321a0a9df98/RELEASES.md#upgrade-instructions) for more details.