Prepare 11.9.0-rc5 release

Release issue: gitlab-org/release/tasks#702 (closed)

Summary

This MR prepares 11-9-stable for 11.9.0-rc5.

Automated cherry-picking

Merge requests labeled ~"Pick into 11.9" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run cherry_pick 11.9.0-rc5