Bump actions/checkout from 6 to 7 - #471

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: OpenModelica/setup-openmodelica@v1.0.7, szenius/set-timezone@v2.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/