Allow epics to have child issues from different group hierarchies
In order to complete &8294 we need to remove model and service
validations allowing epics having child issues from different group hierarchies.
There is a small POC that points out which validations should be removed !95075 (closed).
It is also important to double-check if any information could leak to unauthorized users. There is a small checklist that can be used as inspiration https://gitlab.com/gitlab-org/gitlab/-/issues/370541