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>

debian/cephadm.postinst

@@ -29,8 +29,8 @@ case "$1" in
                  --system \
                  --disabled-password \
                  --home /home/cephadm \
-                 --gecos 'cephadm user for mgr/cephadm' \
                  --shell /bin/bash cephadm 2>/dev/null || true
+         usermod --comment "cephadm user for mgr/cephadm" cephadm
          echo "..done"
        fi