Remove invalid milestoned/demilestoned activity types

pull_request_target does not support these events; milestone changes
are captured by the existing 'edited' trigger.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rparolin

.github/workflows/pr-metadata-check.yml

@@ -12,8 +12,6 @@ on:
       - synchronize
       - labeled
       - unlabeled
-      - milestoned
-      - demilestoned
 
 jobs:
   check-metadata: