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>