MM-56457: Update LastUpdateAt in UpdateMemberNotifyProps (#25984)

PR https://github.com/mattermost/mattermost/pull/18114
mistakenly left out updating the lastUpdateAt of the field.

We fix that here.

https://mattermost.atlassian.net/browse/MM-56457
```release-note
NONE
```

server/channels/store/sqlstore/channel_store.go

@@ -1861,6 +1861,7 @@ func (s SqlChannelStore) UpdateMemberNotifyProps(channelID, userID string, props
 		sql, args, err2 := s.getQueryBuilder().
 			Update("channelmembers").
 			Set("notifyprops", sq.Expr("notifyprops || ?::jsonb", jsonNotifyProps)).
+			Set("LastUpdateAt", model.GetMillis()).
 			Where(sq.Eq{
 				"userid":    userID,
 				"channelid": channelID,
@@ -1884,6 +1885,7 @@ func (s SqlChannelStore) UpdateMemberNotifyProps(channelID, userID string, props
 		sql, args, err2 := s.getQueryBuilder().
 			Update("ChannelMembers").
 			Set("NotifyProps", jsonExpr).
+			Set("LastUpdateAt", model.GetMillis()).
 			Where(sq.Eq{
 				"UserId":    userID,
 				"ChannelId": channelID,