Update GitLab Migration UI to show all groups available to migrate instead of top-level only

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When GitLab Migration is used via UI it only shows top-level groups a user has owner access to, which can be limiting whenever a user wants to migrate a subgroup instead.

It's possible to migrate a subgroup if GitLab Migration API is used.

We should update Gitlab Migration UI to present all groups a user has owner access to, instead of top-level groups only, so that the functionality is identical between UI and API.