[ticket/11169] Move module 'prune users' to users category

PHPBB3-11169
[ticket/11169] Skip migration if "Users" category has been deleted

or the module has already been moved to that category

PHPBB3-11169
Merge remote-tracking branch 'Elsensee/ticket/11169' into develop

* Elsensee/ticket/11169:
  [ticket/11169] Skip migration if "Users" category has been deleted
  [ticket/11169] Move module 'prune users' to users category