Ensure encryption at-rest is enabled for AWS ElasticSearch Domains

AWS ElasticSearch Domains are not encrypted at-rest and may expose sensitive customer data.

In AWS Console -

1. Sign in to AWS Console and go to the Elasticsearch (ES) dashboard.
2. Click on the ES domain.
3. Open the domain configuration page.
4. Check if Encryption at rest is enabled.

In Terraform -

1. In the aws_elasticsearch_domain resource, set the encrypt_at_rest.enabled field to true.

References:
https://docs.aws.amazon.com/opensearch-service/latest/developerguide/encryption-at-rest.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/elasticsearch_domain#encrypt_at_rest