Prepare 13.6.3-ee release

Release issue: gitlab-org/release/tasks#1990 (closed)

Summary

This MR prepares 13-6-stable-ee for 13.6.3-ee.

Automated cherry-picking

Merge requests labeled ~"Pick into 13.6" should be automatically picked into this MR using the following ChatOps command:

# In Slack
/chatops run release merge 13.6.3

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 13-6-stable-ee-patch-3.

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See also Patch Releases documentation.