chore: pin GitHub Actions to commit SHAs - #381

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, cardinalby/schema-validator-action@95ce895b6846cc7d931785fcaa46c84816753612. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/