Pin GitHub Actions to full-length commit SHAs - #682

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-dotnet@871f041373faaad213a635d9afb62905ec029bbb. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/