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

This will allow us to track the state of the backport request in the PR,
rather than in the issue. The state updates for PRs can be automated, so
this will save us some triage work.

Author: tstellar

llvm/utils/git/github-automation.py

@@ -577,6 +577,10 @@ def create_pull_request(self, owner: str, repo_name: str, branch: str) -> bool:
 
             pull.as_issue().edit(milestone=self.issue.milestone)
 
+            # Once the pull request has been created, we can close the
+            # issue that was used to request the cherry-pick
+            self.issue.edit(state="closed", state_reason="completed")
+
             try:
                 self.pr_request_review(pull)
             except Exception as e: