Pin GitHub Actions to full-length commit SHAs - #356

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@a37ce9120846195fa4ece8f58b268e6043cb2f26, actions/setup-dotnet@55ec9447dda3d1cf6bd587150f3262f30ee10815. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/