Update response code to 403

ee/lib/api/alert_management_alerts.rb

@@ -80,7 +80,7 @@ module API
 
           authorize!(:update_alert_management_metric_image, alert)
 
-          render_api_error!('Feature not available', 400) unless alert.metric_images_available?
+          render_api_error!('Feature not available', 403) unless alert.metric_images_available?
 
           metric_image = alert.metric_images.find_by_id(params[:metric_image_id])
 

ee/spec/requests/api/alert_management_alerts_spec.rb

@@ -335,7 +335,7 @@ RSpec.describe API::AlertManagementAlerts do
         it 'returns an error' do
           subject
 
-          expect(response).to have_gitlab_http_status(:bad_request)
+          expect(response).to have_gitlab_http_status(:forbidden)
           expect(json_response['message']).to eq('Feature not available')
         end
       end