GH Actions: Bump actions/checkout from 6.0.2 to 7.0.0 - #74

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: korelstar/xmllint-problem-matcher@1bd292d642ddf3d369d02aaa8b262834d61198c0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/