In commit 357e2ef429 (CheckSoureRuns: Add a unified way to check if a
source runs, 2020-09-14, v3.19.0-rc1~118^2), the default Fortran source
file extension was accidentally changed from `.F90` to `.F`. Fix that.
In commit 10ae907de0 (CheckSoureCompiles: Add a unified way to check if
a source compiles, 2020-09-14, v3.19.0-rc1~118^2~1), the default Fortran
source extension was correctly preserved as `.F`, but `.F90` is a better
default both for consistency and modern usage. Use that for direct
calls to the `check_source_compiles` macro. Update our original
`check_fortran_source_compiles` implementation to use `.F` by default
as it did before.
Fixes: #21307
