Bump actions/checkout from 6.0.3 to 7.0.0 - #418

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: pascalgn/automerge-action@v0.16.4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/