doc: update doc to delete alpha snapshot
doc: update doc to delete alpha snapshot

updated upgrade documentation to remove
the snapshot created by alpha driver before
upgrade of CSI driver as beta snapshot is not
backward compatible with the alpha snapshot.

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

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