ci: render macOS and Windows coverage with gcovr's boost theme - #397

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, alandefreitas/cpp-actions/boost-clone@v1.9.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/