Prepare 15.6.8 release for cng-jh

Release issue: https://jihulab.com/gitlab-cn/release/tasks/-/issues/41

Summary

This MR prepares 15-6-stable-jh for 15.6.8.

Automated cherry-picking

Merge requests labeled ~"Pick into 15.6" should be automatically picked into this MR.

Manual intervention

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

---

See also Patch Releases documentation.