Deprecate publickey(5) stuff

This uses DES and it's likely that nobody uses that in 2025.
If somebody uses this we help them by deprecating and removing this.

Reviewed by: bapt, emaste
Differential Revision: https://reviews.freebsd.org/D30682

(cherry picked from commit 723425f837270dd3b22098168ae9464a1ebe38c6)