Prepare 12.1.3-ee release

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

Summary

This MR prepares 12-1-stable-ee for 12.1.3-ee.

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

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.