Do not export certain models when using Relations Export API

What does this MR do?

Relations export API https://docs.gitlab.com/ee/api/group_relations_export.html exports all Group/Project top level relations that are defined here:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/import_export/group/import_export.yml e.g. epics, labels, milestones
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/import_export/project/import_export.yml e.g. issues, merge reuqests

This API has been introduced to support Group Migration feature (also known as Bulk Imports) https://docs.gitlab.com/ee/user/group/import/ however not all exported relations are needed by Bulk Import functionality, specifically members export, as Bulk Import uses GraphQL API to import members from source to destination Gitlab instance.

This MR adds a list of 'skipped' relations to relations export api, as they are not needed.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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