net/wifi-firmware-kmod: stop building kmods on main and 14.2 and on

In Feb 2024 a feature to firmware(9) was introduced to be able to load
plain firmware files (not kernel modules) from /boot/firmware/ .
If the system is modern enough stop building kernel modules but simply
copy the firmware files in.
This makes the entire build process a lot simpler.
For older versions we still have to generate kernel modules (though
they all shipped with firmware in src so we can possibly unsupport
them in the future).

Sponsored by: The FreeBSD Foundation
Reviewed by: jrm, imp
Differential Revision: https://reviews.freebsd.org/D47068