Update extensions of the Fortran source files

I recommend renaming all Fortran files that require preprocessing from *.f90 to *.F90. Most compilers follow the convention and apply the preprocessing without any additional flags. This might help us with compilers that are not known to CMake. For example, we have to set -fpp for the NEC Aurora compiler nfort (see here). I have not checked this yet but my assumption is that we would not have to set the extra flag if the source files of the library have followed the naming convention. Also, if I remember correctly, one of the reasons for having to require CMake 3.18+ is the Fortran_PREPROCESS property. We could live without it and be fine with even older versions of CMake if we "correct" the filename extensions.