Prepare 15.1.6 release

Release issue: https://jihulab.com/gitlab-cn/release/tasks/-/issues/6

Summary

This MR prepares 15-1-stable-jh for 15.1.6.

Automated cherry-picking

Merge requests labeled Pick into 15.1 should be automatically picked into this MR using the following ChatOps command:

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 15-1-stable-jh-patch-6.

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See also Patch Releases documentation.