this bundles the current package recipe of forgejo in nixpkgs-unstable.
Implies updating forgejo, since nixpkgs-stable is still on 1.20.6 (v6 in
the new version scheme).
This'll mean we have to manually update it same as with mattermost, and
can potentially also help with upstream changes. If we get tired of
that, we can always decide to just use the nixpkgs-unstable version
directly.
