Ensure encryption at rest is enabled for AWS DocumentDB clusters

Data could be exfiltrated due to unencrypted AWS DocumentDB clusters.

In AWS Console -

1. Sign in to AWS Console and go to DocumentDB dashboard.
2. Select Clusters in the navigation pane.
3. Select the AWS DocumentDB cluster to examine,and view the Cluster identifier column.
4. Verify the 'Encryption at-rest' configuration value to check if it is set to 'Yes'.

In Terraform -

1. In the aws_docdb_cluster resource, set the storage_encrypted field to true.

References:
https://docs.aws.amazon.com/documentdb/latest/developerguide/encryption-at-rest.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/docdb_cluster