Prepare 12.3.1-ee release

Release issue: gitlab-org/release/tasks#984 (closed)

Summary

This MR prepares 12-3-stable-ee for 12.3.1-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.3" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run release merge 12.3.1

Once automated cherry-picking has completed, the CE merge request still needs to be merged into this merge request in order to bring in changes that are not EE-specific.

Refer to the documentation for more information.