ci: remove unmaintained k8s versions
ci: remove unmaintained k8s versions

Version 1.18.x is currently out, and the k8s community does not maintain
versions 1.14 and 1.15 anymore. No need to test on versions that users
should not deploy.

Fixes: #1023
Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

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