gfortran.mk: Change fallback version from 7 to 10

gfortran.mk has code to choose the version of gfortran (really, the
version of pkgsrc gcc from which to use gfortran), and the first plan
is to match the gcc version in use.  Only if that version is not
available does it use a fallback version.  That used to be 7, back
when 7 was the standard relatively modern approach.  Today, 10 is the
standard approach for reasonably-up-to-date but not super-new gcc.

As proposed on tech-pkg on with no objections.

(gdt)