Fix EZP-23381: UserService::loadUserGroupsOfUser() checks wrong permissions

> https://jira.ez.no/browse/EZP-23381

Loading user group of a user checks edit permission on the $user. Since loadUserGroupsOfUser is a read-only method, it is more relevant to check for check read-only permissions.

See comment on ca7f938c6ac8637cddbbfb28e6c8acf19c2c36c6