Prepare 15.8.3-ee release

Release issue: gitlab-org/release/tasks#5063 (closed)

Summary

This MR prepares 15-8-stable-ee for 15.8.3-ee.

Automated cherry-picking

Merge requests labeled Pick into 15.8 should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 15.8.3

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 15-8-stable-ee-patch-3.

---

See also Patch Releases documentation.