Pin GitHub Actions to full-length commit SHAs - #178

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@0717577d45739eb3c851188b29f50ed6c0b2194e, actions/setup-node@7c12f8017d5436eb855f1ed4399f037a36fbd9e8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/