[ticket/12008] Update the run time for the prune notifications cron task.

read_notification_last_gc in the config table is not updated at all at the moment. This is resulting in this particular cron task running whenever it can (when the tasks queued before it do not need to run) and thus prevents all other tasks after it from running. This includes the sessions garbage collection which takes care of removing old sessions and updating the user_lastvisit field in the users table (amongst other things).

http://tracker.phpbb.com/browse/PHPBB3-12008