Do not require admin permissions for Project/Group Import user mapping if importing into a group. Use current group members instead.

Currently in order for imported project to map users (based on email address) an importer user has to be admin: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/import_export/members_mapper.rb#L7

While this solution works for a lot of Import cases, sometimes, with shared GitLab instances (like GitLab.com) - using admin permissions can not be possible. Because of that, a migrating user has to involve a lot of parties (like ProServ, Support, Infra teams) to make an imported project have correctly mapped users. This is a very manual and time consuming process. Instead, an alternative, more automated way of having users mapped can be added.

When importing a Project/Group into a Group If an importer user is a Group Owner Do not require admin permissions And only use users that are members of that group when mapping users

Related to #19128 (comment 300134398)