Issue/595 Raise the minimum PHP version to 8.1 and test 8.1, 8.2 and 8.3 in CI - #600

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, codecov/codecov-action@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/