auth: require CEPHX_V2 by default

auth: require CEPHX_V2 by default

It's been almost three years and support is present in all relevant
clients.

From the security perspective, roughly the same could be achieved
with "ceph osd set-require-min-compat-client nautilus", but this is
more user friendly as the client gets ENOTSUP instead of spinning on
"feature set mismatch" faults.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit e5744672dbde2a897f5f4959339472b7b10c5688)