Add a `top-level-only` option for group graphql query
The following discussion from !171359 should be addressed:
- [ ] @arfedoro started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/171359#note_2194762325): (+1 comment)
> **question** (non-blocking) :question:
>
> Do I understand right that, you use `api` instead of `graphql`, only for the sake of `top_level_only` option?
>
> If yes and, as general direction, we are steering away from using an api, what do you think about creating a follow-up to update `graphql` in order to include `top_level_only` option?
issue