debian/control: ceph-mgr-modules-core does not Recommend ceph-mgr-rook anymore

debian/control: ceph-mgr-modules-core does not Recommend ceph-mgr-rook anymore

per https://www.debian.org/doc/debian-policy/ch-relationships.html

> Recommends
>   This declares a strong, but not absolute, dependency.
>
> The Recommends field should list packages that would be found together
> with this one in all but unusual installations.

ceph-mgr-modules-core provides a set of ceph-mgr modules which are
always enabeld. but the rook module enables ceph-mgr to install and
configure a Ceph cluster using Rook. this module is very useful but
it does not have such a strong connection with ceph-mgr-modules-core.
we can always install it separately for using better intergration with
Rook.

See-also: https://tracker.ceph.com/issues/45574
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 814fb28892370bef3337f90edccfd6f9c00bf350)