Prepare 11.9.9-ee release

Release issue: gitlab-org/release/tasks#758 (closed)

Summary

This MR prepares 11-9-stable-ee for 11.9.9-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 11.9" should be automatically picked into both the CE and EE prepration MRs using the following ChatOps command:

# In Slack
/chatops run release merge 11.9.9

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.