Migrate from Gitlab to Forgejo

GitLab is going all-in AI and has generally become very slow and bulky for us to run.
Forgejo is a lot faster and has (almost) all of the features we actually use as well, except for Pages (but we can figure out a solution for that pretty easily).

So, we want to check out if we can move our repo's over!

This requires a multi-step process:

- [x] - Setting up a forgejo instance
- [x] - Configuring the instance to link to our OIDC logins (and groups if possible)
- [x] - ~~(if group sync is not possible) Update/write a claudia plugin to manage users, groups and memberships~~ (see https://codeberg.org/forgejo/forgejo/pulls/11656)
- [ ] - Find a solution for replacing the sites on Pages (static NGINX container build and deploy)
- [ ] - Migrate repositories over from GitLab to Forgejo instance
- [ ] - Update any CI/CD pipelines so they support Forgejo Actions
- [ ] - Decommission Gitlab

Forgejo has their own helm chart we can use: https://code.forgejo.org/forgejo-helm/forgejo-helm
GEANT has a data migration script that we may be able to use: https://codeberg.org/GEANT/gitlab_to_forgejo

Start with the Forgejo helm chart and the GEANT GitLab-to-Forgejo migration script linked in the issue. Map the remaining checklist items: a static NGINX container for Pages, repository migration, Forgejo Actions updates, and GitLab decommissioning. Done means each unchecked migration step has been completed and the resulting setup works.

Written by the indexing model from the issue text.