lang/go: Add bsd.go.mk and use it for go124 and go125

Previously, we used lang/go-devel as a MASTERDIR, but go-devel itself
was really old (essentially 3 years old), and Go doesn't have the
concept of a public "development" version anymore.

Rather than tying modern Go ports to a vestigial version that nobody
should actually use, this commit adds a bsd.go.mk that Go ports can
include. This allows us to get lang/go* Makefiles down to 3 lines.

While here, extend the pkg-descr for both lang/go and the lang/go1*
ports to help users understand what's installing what.

This "new" framework (gently modified from go-devel/Makefile, so all
credit goes to everyone involved in creating that file) provides for
a default pkg-descr, but keeps each port responsible for its own
patches.

No functional changes are intended other than a slightly extended
pkg-descr (not even important enough to bump PORTREVISIONs here). Any
change in behaviour is a bug.