Prepare 13.4.1 release

Release issue: gitlab-org/release/tasks#1692 (closed)

Summary

This MR prepares 13-4-stable for 13.4.1.

Automated cherry-picking

Merge requests labeled ~"Pick into 13.4" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 13.4.1