Detailed metrics modal: Fix modal footer

The modal for detailed metrics shouldn't have a footer with buttons, but
because we use the wrong slot name, `#footer` instead of
`#modal-footer`, we show OK and Cancel Buttons.

See: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46183

app/assets/javascripts/performance_bar/components/detailed_metric.vue

@@ -141,7 +141,7 @@ export default {
         </template>
       </table>
 
-      <template #footer>
+      <template #modal-footer>
         <div></div>
       </template>
     </gl-modal>