Hide the milestone in the API when the feature is not available

lib/api/entities.rb

@@ -619,7 +619,8 @@ module API
       expose :id
       expose :name
       expose :project, using: Entities::BasicProjectDetails
-      expose :milestone
+      expose :milestone,
+             if: -> (board, _) { board.project.feature_available?(:issue_board_milestone) }
       expose :lists, using: Entities::List do |board|
         board.lists.destroyable
       end

spec/requests/api/boards_spec.rb

@@ -57,6 +57,24 @@ describe API::Boards do
         expect(response).to match_response_schema('public_api/v4/boards')
       end
     end
+
+    context 'with the issue_board_milestone-feature available' do
+      it 'returns the milestone when the `issue_board_milestone`-feature is enabled' do
+        stub_licensed_features(issue_board_milestone: true)
+
+        get api(base_url, user)
+
+        expect(json_response.first["milestone"]).not_to be_nil
+      end
+
+      it 'hides the milestone when the `issue_board_milestone`-feature is disabled' do
+        stub_licensed_features(issue_board_milestone: false)
+
+        get api(base_url, user)
+
+        expect(json_response.first["milestone"]).to be_nil
+      end
+    end
   end
 
   describe "GET /projects/:id/boards/:board_id/lists" do