Ensure public access is disabled for AWS Neptune cluster instances

Allowing unrestricted, public access to cloud services could open an application up to external attack. Disallowing this access is typically considered best practice.

In AWS Console -

1. Sign in to AWS Console and go to the Amazon Neptune console.
2. Navigate to Neptune instance and then select your public database.
3. Select Modify.
4. Under Connectivity, expand Additional connectivity configuration.
5. Under Public access, select Not publicly accessible.

In Terraform -

1. In the aws_neptune_cluster resource, set the publicly_accessible field to false.

References:
https://docs.aws.amazon.com/neptune/latest/userguide/get-started-security.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/neptune_cluster_instance#publicly_accessible