Enable ndjson import/export on GitLab.com

Once testing has completed on Staging, we need to roll out ndjson on GitLab.com so users can start to take advantage of all of the improvements.

1. Testing issue: #215059 (closed)
2. Rollout plan: #35861 (closed)

It would be great if we could get this change into 13.0, so we can align it with a major release.

Proposed rollout to GitLab.com

1. groupmemory validate together pairing with ~"group::import" team that ndjson import/export works properly
2. Enable FF by default in 13.0
3. Update documentation #215125 (closed)
4. Disable FF for GitLab.com once the change to enable goes live, until the 22th when the %13.0 is released. We can selectively enable for a few test namespaces to confirm it is working. We don't want to break the ability to import into GitLab.com until 13.0 is officially released.
5. Re-enable FF on GitLab.com on 22th.