Import fails when importing admin users

By default, we import users in the confluence admins group by outputting the XWiki.XWikiAdminGroup.

This fails because of a bug in the user output filter stream (XWIKI-22765), but with this bug fixed, the XWiki.XWikiAdminGroup would be overwritten, which would remove current XWiki admin users, which is quite bad. This has probably never actually worked.

We would need XWIKI-22764 to be able to update the group instead. When we have this possibility, we should probably do this for all the imported groups (and maybe provide an option to overwrite non-admin groups, which would correspond to the current, unsafe, behavior for non admin groups).