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>

module/os/freebsd/spl/spl_kmem.c

@@ -240,22 +240,14 @@ void
 kmem_cache_reap_soon(kmem_cache_t *cache)
 {
 #ifndef KMEM_DEBUG
-#if __FreeBSD_version >= 1300043
 	uma_zone_reclaim(cache->kc_zone, UMA_RECLAIM_DRAIN);
-#else
-	zone_drain(cache->kc_zone);
-#endif
 #endif
 }
 
 void
 kmem_reap(void)
 {
-#if __FreeBSD_version >= 1300043
 	uma_reclaim(UMA_RECLAIM_TRIM);
-#else
-	uma_reclaim();
-#endif
 }
 #else
 void