fix: cross-compile x86_64 on arm64 runner since macos-13 is no longer…

… available

GitHub Actions deprecated the macos-13 runner. Use macos-latest (arm64) with
jurplel/install-qt-action clang_64 to install x86_64 Qt libraries, then set
CMAKE_OSX_ARCHITECTURES=x86_64 for cross-compilation to Intel target.

Update README.md