handbook: OpenSSH no longer supports protocol v1

Base system commit 4f52dfbb8d6c (May 2018) introduced OpenSSH 7.6p1,
which dropped support for the SSH v1 protocol. The v2 protocol has
existed for a very long time and is ubiquitous so there is no need
to mention protocol compatibility at all here.

Reviewed by: ygy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31079