Run PR check action on synchronize (#5140)

When reusing this action elsewhere, I noticed that its status would be
cleared if additional commits were pushed to the PR branch. Triggering
on the synchronize event ensures that it will be checked on subsequent
commits.

.github/workflows/pr-check.yml

@@ -4,7 +4,7 @@
 name: Pull Request Validation
 on:
   pull_request:
-    types: [opened, edited, reopened]
+    types: [opened, edited, reopened, synchronize]
 jobs:
   check-pr-content:
     runs-on: ubuntu-latest