Update macOS and clang versions in workflows

[samhatfield]

macos 13 is deprecated.

[samhatfield]

I don't quite understand the compiler suite on macOS. We are asking for gfortran-13, and according to the CMake output, that's what we're getting for the Fortran compiler. But the C and C++ compilers are simply ~, apparently because Xcode requires empty environment variables. Does that mean we can't control the clang version for these compilers? In any case, we're apparently using clang-17, not 18, so I need to update the label.

[wdeconinck]

Indeed, the C/C++ compiler are provided by the OS / developer tools.
Homebrew package manager is based on this as well.
Fortran is not part of the OS and is installed via homebrew. CMake identifies it as AppleClang 17 so I think clang-17 is correct.
Thanks for spotting!

[samhatfield]

Okay to merge then?