Implement general improvements to the backport and stable branch experience

Problem

There a number of improvements that we can make to the backport and stable branch pipelines.

This serves as a general "catch-all" for these improvements.

Proposal

Gather and make general improvements to the backport and stable branch pipelines.

Tasks

• Enable FF_TIMESTAMPS for stable branch pipelines (gitlab-org/gitlab!190877 - merged)
• For Tier 3 backport MRs, split rubocop CI job into FOSS and EE jobs (similar what we did with haml lint) and run them in parallel (dependent on rubocop RUN_ALL_RUBOCOP=true being added to tier 3) (#21126 (closed))
• Improve performance of RuboCop and its cops (gitlab-org/gitlab#377469) (?)
• Ensure stable prefix tags exist for stable-ee branches. Release flow has changed and current rules do not work anymore due to stable branch being created before release tag is which causes backport mr pipelines fail
• Fix issue where ~"missed:x.y" label would be applied to a backport MRs targeting older milestones.
• Enable pipeline tiering for database changes to provide more granular control of runs
• Add observability into MRs that should be backported.
• Add a script for creating backports

Results

What was done	Result
Added timestamps to job logs of stable branches.	Makes diagnosing pipeline-related problems more efficient for stable branches.