[llvm] workflows: Automatically make backport requests from merged PRs (PR #126993)

[via llvm-commits]

llvmbot wrote:

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>> For some reason the issue-release-workflow triggered on this: https://github.com/llvm/llvm-project/actions/runs/13298256945/job/37134833231
>
>Oh I see, it saw `/cherry-pick ${context.sha}` in the diff in the first comment.

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/126993