Admin settings to enable/disable file-based Group export/import
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### About
Group file-based export/import is currently always enabled.
This is unlike project file-based export/import which can be controlled by admin settings (go to `/admin/application_settings/general`, and expand **Import and export settings**):
* Project _import_ is only enabled when the `GitLab export` import source is enabled ([example](https://gitlab.com/gitlab-org/gitlab/-/blob/a8f96f102874e7e57c034a83d2c299509c8a9328/lib/api/project_import.rb#L42))
* Project _export_ is only enabled when the `Project export enabled` setting is enabled ([example](https://gitlab.com/gitlab-org/gitlab/-/blob/a8f96f102874e7e57c034a83d2c299509c8a9328/lib/api/project_export.rb#L14))
### Proposal
Have group file-based export/import observe the _same settings_ as project file-based export/import.
1. Update the UI and the API to check these same settings for file-based group export/import (we can refer to project export/import UI and API for where to update for groups).
1. Rename the `Project export enabled` checkbox in the admin UI to `GitLab file exports enabled`
1. We could possibly rename `GitLab export` import source in the UI to be `GitLab file exports` at the same time
2. Update documentation
issue