x11-wm/fvwm3: Update to 1.0.6a

Also switch to PORTVERSION because 1.0.6a is actually newer than 1.0.6
and patch configure.ac to avoid pinning the go version and to drop the
compiler warning test that's incompatible with clang 15.

Changelog:
https://github.com/fvwmorg/fvwm3/releases/tag/1.0.6
https://github.com/fvwmorg/fvwm3/releases/tag/1.0.6a

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D38695