Update MediaWiki:Communitymessages-notice-msg

MediaWiki:Communitymessages-notice-msg is used to inform people of news on a wiki by providing a link to Special:ActivityFeed (now Special:WikiActivity) by default. However, now that Special:ActivityFeed apparently does not redirect to Special:WikiActivity anymore, I would recommend editing the Community messages notice on your wikis so the link takes you to Special:WikiActivity.