Add option to disable Group Export
### Problem to solve
There is currently an option within the Admin (Settings > General > Visibility and access controls) to disable project exports, but no equivalent for disabling group exports.
### User experience goal
Adding a group export will keep the access controls consistent.
### Proposal
Add another option to disable the Group export alongside the project export setting.

### Links / references
[MR for Project Setting](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/13211)
[Internal ZD](https://gitlab.zendesk.com/agent/tickets/159350)
issue