Fix accessibility label for "start a new discussion"

app/assets/javascripts/diffs/components/diff_discussion_reply.vue

@@ -36,6 +36,7 @@ export default {
       <template v-else-if="renderReplyPlaceholder">
         <reply-placeholder
           :placeholder-text="__('Start a new discussion…')"
+          :label-text="__('New discussion')"
           @focus="$emit('showNewDiscussionForm')"
         />
       </template>

app/assets/javascripts/notes/components/discussion_reply_placeholder.vue

 <script>
+import { __ } from '~/locale';
+
 export default {
   name: 'ReplyPlaceholder',
   props: {
     placeholderText: {
       type: String,
-      required: true,
+      required: false,
+      default: __('Reply…'),
+    },
+    labelText: {
+      type: String,
+      required: false,
+      default: __('Reply to comment'),
     },
   },
 };
@@ -16,7 +24,7 @@ export default {
     rows="1"
     class="reply-placeholder-text-field js-vue-discussion-reply"
     :placeholder="placeholderText"
-    :aria-label="__('Reply to comment')"
+    :aria-label="labelText"
     @focus="$emit('focus')"
   ></textarea>
 </template>