Prepare 15.5.4-ee release

Release issue: gitlab-org/release/tasks#4523 (closed)

Summary

This MR prepares 15-5-stable-ee for 15.5.4-ee.

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 15.5.4

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 15-5-stable-ee-patch-4.

---

See also Patch Releases documentation.