chore: pin GitHub actions to commit SHAs and add dependabot - #31

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020, denoland/setup-deno@61fe2df320078202e33d7d5ad347e7dcfa0e8f31. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/