chore: Use shared validate-pr composite action (#5240)

Replace the inline PR validation workflow with the shared composite action
from getsentry/github-workflows#153.

This shrinks the workflow from 300+ lines to ~15 and ensures future updates
to validation logic are picked up automatically.

#skip-changelog

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