Prepare 12.1.3 release

Release issue: gitlab-org/release/tasks#888 (closed)

Summary

This MR prepares 12-1-stable for 12.1.3.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.1" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run release merge 12.1.3