doc: lift the minimum supported version of ceph to v15.0.0
doc: lift the minimum supported version of ceph to v15.0.0

at present the supported ceph cluster version has been listed with
v14.x.x version of Ceph. However upstream Ceph has dropped its
support for 14.x version and minimum supported release is v15.x

Ref # https://docs.ceph.com/en/quincy/releases/index.html

this commit update the ceph cluster version mention to v15.0.0

Additional Note:
The code should work even for last archived release but we dont
need that mention anymore in the readme.

Signed-off-by: Humble Chirammal <dc8ba7e82a7df22725dedec9b10c7aaab9ec713f@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi