gdstk backend: close API gaps so the tutorials run - #104

❌ Failed to create checks using the provided token. (HttpError: Resource not accessible by integration - https://docs.github.com/rest/checks/runs#create-a-check-run)

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4, actions/upload-artifact@v4, mikepenz/action-junit-report@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/