Add a notice when the label does nto exist.

Author: desrosj

.github/workflows/backport.yml

@@ -287,4 +287,6 @@ jobs:
 
           if gh label list --repo "${PR_REPO}" --json name --jq '[.[].name] | contains(["Auto-backport"])' | grep -q 'true'; then
             gh pr edit "$PR_URL" --repo "${PR_REPO}" --add-label 'Auto-backport'
+          else
+            echo "::notice::The 'Auto-backport' label does not exist on ${PR_REPO}. Consider adding it so that backport pull requests can be identified easily."
           fi