Backport 3 commits for Merge Train pipelines support in 17-0-stable-ee

What does this MR do and why?

This backports 6 MRs to the 17-0-stable-ee branch:

1. ci: Add a new pre-merge-checks job (!154540 - merged)
2. Streamline & fix merge train pipeline (!154934 - merged)
3. Find the first non-merge train pipeline in pre-... (!154945 - merged)
4. ci: Ensure rules checking MR labels don't run o... (!155048 - merged)
5. Change pipeline freshness threshold from 4 to 8... (!155226 - merged)
6. Ensure latest pipeline before merge train passed (!155604 - merged)

This is needed otherwise the .if-automated-merge-request CI rule would run in Merge Train pipelines because the target branch matches /stable-(ee|jh)$/ (that's not the only problem, but that was the one discovered in gitlab-org/quality/engineering-productivity/approved-mr-pipeline-incidents#496 (closed)).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).