Prepare 13.9.1-ee release

Release issue: gitlab-org/release/tasks#2265 (closed)

Summary

This MR prepares 13-9-stable-ee for 13.9.1-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 13.9" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 13.9.1

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 13-9-stable-ee-patch-1.

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See also Patch Releases documentation.