UX: add heading role to close composer dialog (#29590)

app/assets/javascripts/discourse/app/components/modal/discard-draft.gjs

@@ -24,7 +24,7 @@ export default class DiscardDraftModal extends Component {
       @dismissable={{false}}
     >
       <:body>
-        <div class="instructions">
+        <div class="instructions" role="heading" aria-level="1">
           {{i18n "post.cancel_composer.confirm"}}
         </div>
       </:body>