Close merge requests for closed issues

Problem/Motivation

GitLab does some background work for every open merge request after every commit. This scales linearly and takes ~90 minutes currently. When these stack up with 3 or 4+ commits within 90 minutes, this saturates GitLab’s internal queues with the subsequent jobs queued by this process.

Core has thousands of MRs for closed issues that can be closed. Either from approaches not taken, or the merge request that was applied manually. Core committers should not rate limit commits or really need to be concerned about closing everything.

https://git.drupalcode.org/project/infrastructure/-/work_items/3606934 should mitigate this for git.drupalcode.org using queue routing. The same queued work will still happen and stack up, but hopefully will avoid delays for queued items that people expect to be near-real-time.

Proposed resolution

Automate the closing. This is an issue unique to core, so it should be a CI job that the core project runs, not infrastructure.

Remaining tasks

• Decide on a policy for auto-closing, wording for any comment made on auto-close.
• Implement it.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet