[llvm] [workflows] Close issues used for backports once the PR has been created (PR #80394)

[Tobias Hieta via llvm-commits]

https://github.com/tru commented:

Hm, before we have had the issue open until we had merged the PR. In case some of the needs some further discussion and revisions. Should we just do those comments and revisions directly in the PR going forward?

https://github.com/llvm/llvm-project/pull/80394