Expose updated_at property for cluster env

This fixes the cluster environments page showing the wrong value for
"Last updated".

ee/app/serializers/clusters/environment_entity.rb

@@ -14,6 +14,8 @@ module Clusters
 
     expose :rollout_status, if: -> (*) { can_read_cluster_deployments? }, using: ::RolloutStatusEntity
 
+    expose :updated_at
+
     private
 
     alias_method :environment, :object

ee/spec/fixtures/api/schemas/clusters/environment.json

@@ -20,7 +20,8 @@
     },
     "rollout_status": {
       "$ref": "../rollout_status.json"
-    }
+    },
+    "updated_at": { "type": "date" }
   },
   "additionalProperties": false
 }

ee/spec/serializers/clusters/environment_entity_spec.rb

@@ -34,6 +34,10 @@ describe Clusters::EnvironmentEntity do
       expect(subject).to include(:environment_path)
     end
 
+    it 'exposes updated_at' do
+      expect(subject).to include(:updated_at)
+    end
+
     context 'deploy board available' do
       before do
         allow(group).to receive(:feature_available?).and_call_original