Prepare 15.8.6 release for cng-jh

Release issue: https://jihulab.com/gitlab-cn/release/tasks/-/issues/53

Summary

This MR prepares 15-8-stable-jh for 15.8.6.

Automated cherry-picking

Merge requests labeled ~"Pick into 15.8" should be automatically picked into this MR.

Manual intervention

If merge requests cannot be picked automatically they have to be created manually targeting 15-8-stable-jh-patch-6.

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See also Patch Releases documentation.