Prepare 12.6.1-ee release

Release issue: gitlab-org/release/tasks#1121 (closed)

Summary

This MR prepares 12-6-stable-ee for 12.6.1-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 12.6" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 12.6.1