Deprecate support for Dell EMC VNX driver

The Dell EMC VNX driver is deprecated because of the product reached
its EOL.

Depends-on: https://review.opendev.org/c/openstack/manila/+/907370
Change-Id: Ib4de7371964ea0e25753ec04e338480039a45355

manifests/backend/dellemc_vnx.pp

@@ -1,5 +1,6 @@
 # == define: manila::backend::dellemc_vnx
 #
+# DEPRECATED !!
 # Configures Manila to use the Dell EMC Isilon share driver
 # Compatible for multiple backends
 #
@@ -121,6 +122,8 @@ define manila::backend::dellemc_vnx (
   include manila::deps
   include manila::params
 
+  warning('Support for Dell EMC VNX driver has been deprecated.')
+
   if $driver_handles_share_servers != undef {
     warning('The driver_handles_share_servers parameter has been deprecated and has no effect')
   }