Mk/bsd.port.mk: mark FreeBSD 13.4 as unsupported.

Drop support for FreeBSD 13.4 (and sometimes 12, 14.0, 14.1) from all
ports.

Reviewed by: dch, @gor_clogic.com.ua
Approved by: portmgr (implicit)
Differential Revision: https://reviews.freebsd.org/D50769