net/libpfctl: add 15.1, remove 13.5

13.5 is end-of-life, remove it from this port.
Add 15.1, which is in the process of being released.

Deliberately do not bump PORTVERSION, because we don't change existing
libpfctl versions, we only add a new supported FreeBSD version.

PR: 294871
Sponsored by: Rubicon Communications, LLC ("Netgate")