[llvm] github-automation: Use the llvm/llvm-project repo for backport pull requests (PR #71727)

[Tobias Hieta via llvm-commits]

tru wrote:

> @tru This is ready for review/merge now.

Hmm. Yeah - I wonder if we should allow for a little more time to make sure we don't want to do an emergency 17.x release. I guess we still could but we would have to ad-hoc the cherry-picking in that case. I guess I am fine with that as well, we just need to keep that in mind.

https://github.com/llvm/llvm-project/pull/71727