Prepare 12.9.4 release

Release issue: gitlab-org/release/tasks#1295 (closed)

Summary

This MR prepares 12-9-stable for 12.9.4.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.9" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 12.9.4