net/libpfctl: support building on 13.2

While here also remove references for 12.3 and 13.1.

Reviewed by: garga
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41297

(cherry picked from commit e23e7c9a19fecfad2758e39890d6f0d93563f3ce)