ssh: update sshd_config for prohibit-password option

The PermitRootLogin option "prohibit-password" was added as a synonym
for "without-password" in 2015. Then in 2017 these were swapped:
"prohibit-password" became the canonical option and "without-password"
became a deprecated synonym (in OpenSSH commit 071325f458).

The UsePAM description in sshd_config still mentioned
"without-password." Update it to match the new canonical option.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week