Deprecate support of MemcachedCache backend

This backend was deprecated in django 3.2 and was removed in 4.1.

At the time of writing, all of the supported distributions provide
django 3.2 which contains the deprecation, so we can deprecate its
support now.

Change-Id: Ic0781aa5cd2100ded69ec262cffd834b0de86495

manifests/init.pp

@@ -667,6 +667,8 @@ class horizon(
 
   if $manage_memcache_package {
     if $cache_backend =~ /\.MemcachedCache$/ {
+      warning("Support for MemcachedCache backend has been deprecated. \
+Use PyMemcacheCache backend instead")
       ensure_packages('python-memcache', {
         name => $::horizon::params::memcache_package,
         tag  => ['openstack'],

releasenotes/notes/deprecate-memcache-backend-ce0b3304e4842cae.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    Support for the MemcachedCache backend has been deprecated. This backend
+    was deprecated in django 3.2 .