Change suffix of .f free-form source files to .F90/.f90

[climbfuji]

For transitioning to cap_gen.py, we must be using the correct filename extension for free-form and fixed-form variables.

I think that currently there are a number of .f files that are free-form and that have hard-coded compiler flags to make this switch. I understand from PR NCAR/ccpp-framework#355 that this is not supported.

In any case, cleaning this up is a good thing.

[dustinswales]

@climbfuji Not sure if this is still valid?
As a reference, I didn't run into any issue with capgen and the file extensions when running the SCM with the GFS v16 suite.

[climbfuji]

We should look at the .f files in the repo and see if they are manually promoted to free-form because of harcoded compiler switches (which you probably were using with the SCM).