FIX: ensures updateLastRead is called when receiving a message (#20728)

This behavior is hard to test as it's mostly fixing a race condition: User A sends a message at the same time than User B, which as a result doesn't cause a scroll for the second message and we don't update last read unless we do a small up and down scroll.

`updateLastRead` is debounced so it has no direct consequences to call it slightly more often than what should ideally be needed.

plugins/chat/assets/javascripts/discourse/components/chat-live-pane.js

@@ -613,6 +613,7 @@ export default class ChatLivePane extends Component {
       const message = ChatMessage.create(this.args.channel, data.chat_message);
       this.args.channel.addMessages([message]);
       this.scrollToLatestMessage();
+      this.updateLastReadMessage();
     } else {
       // If we are almost at the bottom, we append the message and notice the user
       const message = ChatMessage.create(this.args.channel, data.chat_message);