fix: Mentions stop working when mentioned user changes username

.changeset/large-files-agree.md

@@ -0,0 +1,5 @@
+---
+"@rocket.chat/meteor": patch
+---
+
+Fixed mentions not working when mentioned user changes username.

apps/meteor/server/models/raw/Messages.ts

@@ -1245,6 +1245,9 @@ export class MessagesRaw extends BaseRaw<IMessage> implements IMessagesModel {
 				'mentions.$.username': newUsername,
 				'msg': newMessage,
 			},
+			$unset: {
+				md: 1,
+			},
 		};
 
 		return this.updateOne(query, update);