Return Vulnerabilities::Feedback created_at column

This will be used by the frontend to indicate how long ago an action was
taken

ee/app/serializers/vulnerabilities/feedback_entity.rb

@@ -5,6 +5,7 @@ class Vulnerabilities::FeedbackEntity < Grape::Entity
   include GitlabRoutingHelper
 
   expose :id
+  expose :created_at
   expose :project_id
   expose :author, using: UserEntity
   expose :pipeline, if: -> (feedback, _) { feedback.pipeline.present? } do

ee/spec/fixtures/api/schemas/vulnerability_feedback.json

@@ -10,6 +10,7 @@
   ],
   "properties" : {
     "id": { "type": "integer" },
+    "created_at": { "type": "date" },
     "project_id": { "type": "integer" },
     "author": { "$ref": "../../../../../spec/fixtures/api/schemas/entities/user.json" },
     "pipeline": {

ee/spec/serializers/vulnerabilities/feedback_entity_spec.rb

@@ -10,7 +10,7 @@ describe Vulnerabilities::FeedbackEntity do
   describe '#as_json' do
     subject { entity.as_json }
 
-    it { is_expected.to include(:project_id, :author, :category, :feedback_type) }
+    it { is_expected.to include(:created_at, :project_id, :author, :category, :feedback_type) }
   end
 
   context 'when issue is present' do