nixos/version: validate system.stateVersion (#379754)

  - Overriding Wayland compositor is possible using `enableWaylandSession` option, but you might need to take care [`xfce4-session`](https://gitlab.xfce.org/xfce/xfce4-session/-/merge_requests/49), [`dbus-update-activation-environment`](https://github.com/labwc/labwc/blob/eaf11face68ee1f1bcc7ce1498304ca8c108c8ba/src/config/session.c#L234) and [`systemctl --user import-environment`](https://github.com/labwc/labwc/blob/eaf11face68ee1f1bcc7ce1498304ca8c108c8ba/src/config/session.c#L239) on startup.

  - For new Xfce installations, default panel layout has [changed](https://gitlab.xfce.org/xfce/xfce4-panel/-/merge_requests/158/diffs) to not include external panel plugins by default. You can still add them yourself using the "Panel Preferences" dialog.

- [`system.stateVersion`](#opt-system.stateVersion) is now validated and must be in the `"YY.MM"` format, ideally corresponding to a prior NixOS release.

- GOverlay has been updated to 1.2, please check the [upstream changelog](https://github.com/benjamimgois/goverlay/releases) for more details.

- [`services.mongodb`](#opt-services.mongodb.enable) is now compatible with the `mongodb-ce` binary package. To make use of it, set [`services.mongodb.package`](#opt-services.mongodb.package) to `pkgs.mongodb-ce`.

    toLower

    optionalString

    literalExpression

    match

    mkRenamedOptionModule

    mkDefault

    mkOption

  config = {

    assertions = [

      {

        assertion = match "[0-9]{2}\\.[0-9]{2}" config.system.stateVersion != null;

        message = ''

          ${config.system.stateVersion} is an invalid value for 'system.stateVersion'; it must be in the format "YY.MM",

          which corresponds to a prior release of NixOS.

          If you want to switch releases or switch to unstable, you should change your channel and/or flake input URLs only.

          *DO NOT* touch the 'system.stateVersion' option, as it will not help you upgrade.

          Leave it exactly on the previous value, which is likely the value you had for it when you installed your system.

          If you're unsure which value to set it to, use "${

            if match "[0-9]{2}\\.[0-9]{2}" options.system.stateVersion.default != null then

              options.system.stateVersion.default

            else

              options.system.nixos.release.default

          }" as a default.

        '';

      }

    ];

    system.nixos = {

      # These defaults are set here rather than up there so that

      # changing them would not rebuild the manual