Automate post-release GitLab updates, using gitlab-triage (#17589)

We will use gitlab-triage to solve other problems soon
(https://salsa.debian.org/tails-team/gitlab-migration/-/issues/54),
and it's the perfect tool for the job, so it makes sense
to start using it here as well.

Since gitlab-triage is not in Debian, in order to be compliant with the security
policies applicable to Release Managers:

 - run gitlab-triage in Docker
 - ensure we always use the latest version of gitlab-triage,
   in a fresh Docker image based on the latest version of debian:stable