Add more Fortran compilers in CI

[bonachea]

Currently PRs only test CI against gfortran-13/gcc-13.

This runs the significant risk that gfortran-13 might silently accept non-compliant Fortran code that would be rejected by other, less-permissive compilers. The same is definitely true for gcc-13. There's also a chance that our tests or library might accidentally encode a dependence on an unspecified behavior in a way that is specific to gfortran (e.g. underspecified details about the CFI descriptor behavior).

Given the point of our library is to be compiler-independent, we should endeavor to add as many different fortran compiler families and versions to CI as is feasible, so that we can learn about changes that might impact portability as they occur.