Prepare 13.12.12 release

Release issue: gitlab-org/release/tasks#2921 (closed)

Summary

This MR prepares 13-12-stable for 13.12.12.

Automated cherry-picking

Merge requests labeled Pick into 13.12 should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 13.12.12

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 13-12-stable-patch-12.

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See also Patch Releases documentation.