Prepare 11.8.0-rc6 release

Release issue: gitlab-org/release/tasks#679 (closed)

Summary

This MR prepares 11-8-stable for 11.8.0-rc6.

Automated cherry-picking

Merge requests labeled ~"Pick into 11.8" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run cherry_pick 11.8.0-rc6