Pin GitHub Actions references to full commit SHAs via Renovate.
This ensures supply-chain security by preventing floating tags
from silently pointing at different commits.
Mirrors the change being made in the org-level renovate-config.json
(.github PR #63). Applying it directly here ensures new plugins
created from this template have SHA pinning from day one.
Related: PRI-731
Co-Authored-By: Paperclip <noreply@paperclip.ing>