Maintain project and group file-based import/export as a workaround for migrations over air-gapped networks and to serve other use cases
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## Problem to Solve
GitLab migration replaces the old import / export migration, however some users with [offline networks](https://gitlab.com/groups/gitlab-org/-/epics/4619#note_738548858) can't use the Group Migration because they don't have network connectivity between GitLab instances. They can only work with the export / import way. Also, the Infrastructure team relies on export/import for [data recovery](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/7122#note_967379458).
## Proposed Solution
1. Remove the UI deprecation message ([discussion](https://gitlab.com/gitlab-org/gitlab/-/issues/341282#note_2838863673)). Consider leaving a warning for users that [DT](https://docs.gitlab.com/user/group/import/) is the preferred method.
1. Hide the Import / export functionality behind an Operational Flag, so Self-managed customers who needs to use it will enable it, and will have both the UI and API functionality.
## Use cases
- Exporting projects for compliance reasons. In [example](https://gitlab.com/gitlab-com/dev-sub-department/section-dev-request-for-help/-/issues/165#customer-impact) customer uses [schedule an export](https://docs.gitlab.com/ee/api/project_import_export.html#schedule-an-export) API endpoint.
issue