debian/cephadm.postinst: stop using adduser --gecos

debian/cephadm.postinst: stop using adduser --gecos

--gecos option of adduser is deprecated in debian/bookworm, and
will be removed in debian/trixie,
see https://manpages.debian.org/bookworm/adduser/adduser.8.en.html.
so to be future-proof, let's switch to `usermod --comment`. please
note, since we still need to support ubuntu/jammy which is used in
our CI, and `adduser` shipped by ubuntu/jammy does not support
`--comment` yet, so we cannot use this option.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>