Ensure storage encryption is enabled for AWS Neptune cluster

Not encrypting the AWS Neptune cluster storage can impact the confidentiality of data.

In AWS Console -

1. Sign in to AWS Console and go to the Neptune console.
2. Select the Neptune DB instance.
3. Select 'Enable Encryption' and make sure it says 'Yes'.

In Terraform -

1. In the aws_neptune_cluster resource, set 'storage_encrypted' to 'true'.

References:
https://docs.aws.amazon.com/neptune/latest/userguide/data-protection.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/neptune_cluster