FIX: Invalid date when sending chat message in thread (#24455)

Fixes a minor issue where "Invalid date" is shown briefly
when sending a message in a chat thread. Change to use
`new Date()` instead like the channel staged message which
does not have this issue.

plugins/chat/assets/javascripts/discourse/models/chat-thread.js

@@ -63,7 +63,7 @@ export default class ChatThread {
     message.staged = true;
     message.processed = false;
     message.draft = false;
-    message.createdAt ??= moment.utc().format();
+    message.createdAt = new Date();
     message.thread = this;
 
     this.messagesManager.addMessages([message]);