.github/scripts/auto-backport.py: update method to get closed prs

`commit.get_pulls()` in PyGithub returns pull requests that are directly associated with the given commit

Since in closed PR. the relevant commit is an event type, the backport
automation didn't get the PR info for backporting

Ref: https://github.com/scylladb/scylladb/issues/18973

Closes scylladb/scylladb#21468

(cherry picked from commit ef104b7b96)

Closes scylladb/scylladb#21483

.github/scripts/auto-backport.py

@@ -150,7 +150,10 @@ def main():
         start_commit, end_commit = args.commits.split('..')
         commits = repo.compare(start_commit, end_commit).commits
         for commit in commits:
-            for pr in commit.get_pulls():
+            match = re.search(rf"Closes .*#([0-9]+)", commit.commit.message, re.IGNORECASE)
+            if match:
+                pr_number = int(match.group(1))
+                pr = repo.get_pull(pr_number)
                 closed_prs.append(pr)
     if args.pull_request:
         start_commit = args.head_commit