ci: Update getsentry/github-workflows to 3.4.0 for validate-pr (#5496)

The validate-pr action was pinned to 3.3.0 while the other workflows
in this repo already use 3.4.0. Pin it to the same SHA and add the
matching version comment.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: runningcode

.github/workflows/validate-pr.yml

@@ -10,7 +10,7 @@ jobs:
     permissions:
       pull-requests: write
     steps:
-      - uses: getsentry/github-workflows/validate-pr@26f565c05d0dd49f703d238706b775883037d76b
+      - uses: getsentry/github-workflows/validate-pr@607fed74f812e69201531a5185b6c3c57caa4e89 # v3
         with:
           app-id: ${{ vars.SDK_MAINTAINER_BOT_APP_ID }}
           private-key: ${{ secrets.SDK_MAINTAINER_BOT_PRIVATE_KEY }}