ci: Add a new pre-merge-checks job

What does this MR do and why?

This implements a new pre-merge-checks job that runs on merge train pipelines (this is the only job that should run there).

This jobs checks the type and freshness of the latest MR pipeline (before the merge train one).

This implements the idea from gitlab-org/quality/engineering-productivity/team#510 (comment 1924669215).

Example of such job at https://gitlab.com/gitlab-org/quality/engineering-productivity/merge-train-playground/-/jobs/6966977724 (experimented with gitlab-org/quality/engineering-productivity/merge-train-playground!1 (merged)).

Before merge

• Are we sure this will be the only merge train pipeline job?
  • I think we should be good since we anticipated merge trains by using the $CI_MERGE_REQUEST_EVENT_TYPE == "merged_result" || $CI_MERGE_REQUEST_EVENT_TYPE == "detached" condition for MR pipelines (excluding merge train ones).
  • The only rules where we would run a job in the merge train pipeline was .build-images:rules:retag-gdk-image and I fixed it in this MR.
• Stable releases?
  • We can just "Merge immediately" for these as it would derail the train but it shouldn't be a big deal given that the job is very fast anyway
• What about dev.gitlab.org?
  • It runs a CE instance, so merge train isn't available, but it's fine as we don't use merge requests there anyway.
• What about security mirror?
  • Merge train should also be enabled there but if it's not, it's not a problem either.
• What about merge requests that don't get a pipeline:tier-x label? e.g. when an MR touches only %r{\A(?:docs?|qa|\.gitlab/(issue|merge_request)_templates)/}, or for the Gitaly-update MRs, or for backport MRs.
  • It should work because:
    • If the pipeline has tier:x, we check that it's tier:3
    • Otherwise, we fail if it contains predictive, and pass otherwise so that QA/docs-only/backports aren't checked for tier

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