The cron scheduler might pick a different task but we don't want to go
through them all to find the one we need. So we just make sure cron is
run at all and then manually run the one we want.
PHPBB3-13232
All columns were renamed from having prune_shadow_topics as namebase to just
prune_shadow.
A missing column was also added to the migration file's remove_schema() method.
PHPBB3-12150
