chore: fix auto-approve workflow (aws#13392)

I believe the reason our auto-approve workflow does not work is because it was triggered when the pull request was _created_ and before the `pr/auto-approve` label was applied to it.

This adds `types: [ labeled ]` so the workflow is triggered when a PR is labeled. The condition remains the same.

.github/workflows/auto-approve.yml

@@ -1,7 +1,9 @@
 # Approve PRs with "pr/auto-approve". mergify takes care of the actual merge.
 
 name: auto-approve
-on: pull_request
+on:
+  pull_request:
+    types: [ labeled, unlabeled, opened, synchronize, reopened, ready_for_review, review_requested ]
 
 jobs:
   auto-approve: