Ensure node-to-node encryption is enabled for AWS ElasticSearch Domains

AWS ElasticSearch Domains have node-to-node encryption disabled which may expose sensitive customer data.

In AWS Console -

1. Sign in to the AWS Console and open the OpenSearch Console.
2. Under Managed clusters in the navigation bar, select Domains.
3. Choose the domain to edit, and under the Actions drop-down, select Edit security configuration.
4. Under Encryption, check the box for Node-to-node encryption.
5. Select Save changes.

In Terraform -

1. In the aws_elasticsearch_domain resource, set the node_to_node_encryption block to have the enabled field set to true.

References:
https://docs.aws.amazon.com/opensearch-service/latest/developerguide/ntn.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticsearch_domain#node_to_node_encryption