Remove `update-tag` event and use existing click event, do not access vuejs...

Remove `update-tag` event and use existing click event, do not access vuejs elements directyl instead use `querySelector`

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36753#note_383620360
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36753#note_383620356
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36753#note_383620351

app/assets/javascripts/vue_shared/components/markdown/header.vue

@@ -90,7 +90,6 @@ export default {
     },
     handleQuote() {
       const documentFragment = getSelectedFragment();
-      const area = this.$parent.$slots.textarea[0].elm;
 
       if (!documentFragment || !documentFragment.textContent) {
         this.tag = '> ';
@@ -99,6 +98,7 @@ export default {
       this.tag = '';
 
       const transformed = CopyAsGFM.transformGFMSelection(documentFragment);
+      const area = this.$el.parentNode.querySelector('textarea');
 
       CopyAsGFM.nodeToGFM(transformed)
         .then(gfm => {
@@ -136,7 +136,7 @@ export default {
             :tag="tag"
             :button-title="__('Insert a quote')"
             icon="quote"
-            @update-tag="handleQuote"
+            @click="handleQuote"
           />
         </div>
         <div class="d-inline-block ml-md-2 ml-0">

app/assets/javascripts/vue_shared/components/markdown/toolbar_button.vue

@@ -65,7 +65,7 @@ export default {
     type="button"
     class="toolbar-btn js-md"
     data-container="body"
-    @click="$emit('update-tag'), () => $emit('click')"
+    @click="() => $emit('click')"
   >
     <icon :name="icon" />
   </button>