Prepare 15.5.6 release

Release issue: gitlab-org/release/tasks#4635 (closed)

Summary

This MR prepares 15-5-stable for 15.5.6.

Automated cherry-picking

Merge requests labeled Pick into 15.5 should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 15.5.6

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 15-5-stable-patch-6.

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See also Patch Releases documentation.