WIP: Prepare 12.2.0-rc31 release

Release issue: gitlab-org/release/tasks#921 (closed)

Summary

This MR prepares 12-2-stable for 12.2.0-rc31.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.2" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run release merge 12.2.0-rc31