ssh: Update to OpenSSH 9.6p1

From the release notes,

> This release contains a number of security fixes, some small features
> and bugfixes.

The most significant change in 9.6p1 is a set of fixes for a newly-
discovered weakness in the SSH transport protocol. The fix was already
merged into FreeBSD and released as FreeBSD-SA-23:19.openssh.

Full release notes at https://www.openssh.com/txt/release-9.6

Relnotes: Yes
Sponsored by: The FreeBSD Foundation