Prepare 12.3.4-ee release

Release issue: gitlab-org/release/tasks#999 (closed)

Summary

This MR prepares 12-3-stable-ee for 12.3.4-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.3" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 12.3.4