Prepare the migration to Tor's GitLab

In https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-73-tails-infra-merge-roadmap#timeline the merge is planned for 2027. The Tails Team sees many benefits in this migration happening ASAP, so in 2026 we want to anticipate and prepare as much as we can, on our own, so that once TPA has capacity for this, the migration is as swift as possible.