Mk/bsd.gcc.mk: Switch USE_GCC=14 to new release port

GCC 14 first release is out and the new corresponding lang/gcc14 port
has been created with commit 4700c3f17859f7cc2c00fd5c9c7bf41e92c8142b.

Thus switch USE_GCC=14 from using lang/gcc14-devel to using lang/gcc14.

Approved by: gerald (maintainer)