chore(ci): Fix concurrency key for require-milestone (#1607)

Author: Tobbe

.github/workflows/require-milestone.yml

@@ -9,7 +9,7 @@ on:
 # Cancel in-progress runs of this workflow.
 # See https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-only-cancel-in-progress-jobs-or-runs-for-the-current-workflow.
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.event.pull_request.number }}
   cancel-in-progress: true
 
 permissions: