Terraform is broken because volume endpoints are not using graphql anymore

[DAlperin]

https://community.fly.io/t/fresh-produce-volumes-endpoints-in-machines-api/14408/7?u=lubien

[jeffdeville]

This unblocked me, but I just took @DAlperin's work, and reduced the scope of his PR to this. https://github.com/fly-apps/terraform-provider-fly/pull/251