Use `BatchModelLoader` for GroupType

What does this MR do?

As outlined in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999#note_172098317, we should be using BatchModelLoader for the parent field in GroupType for performance reasons.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Closes #62066 (closed)