FreeBSD: remove support for FreeBSD < 13.0-RELEASE (#16372)
This includes the last 12.x release (now EOL) and 13.0 development
versions (<1300139).
Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>