doc/man: Remove stale EOL release names from deprecation notices

doc/man: Remove stale EOL release names from deprecation notices

ceph.rst: "osd create" deprecation notice cited "the Luminous release"
(2017, EOL 2020). Update to a plain deprecation statement directing
users to the replacement command (osd new).

rbd.rst: cephx_require_signatures option deprecation cited "the Bobtail
release" (2013, EOL 2015) as context for why the option is deprecated.
Remove the EOL release name; retain the deprecation warning. Fix the
companion nocephx_require_signatures notice for consistency ("in a
future release" instead of "in the future").

Fixes: https://tracker.ceph.com/issues/77191
Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>