Ensure database instances with an AWS Aurora cluster should have same accessibility

All database instances within an AWS Aurora cluster does not have the same accessibility.

In AWS Console -

1. Sign in to the AWS Console and open the RDS Console.
2. Under Databases, choose the cluster instance you wish to edit.
3. Select Modify.
4. Under Connectivity, expand the Additional configuration section.
5. Set the Public Access option to Not publicly accessible.

In Terraform -

1. For each aws_rds_cluster_instance resource where the publicly_accessible field is false, set the publicly_accessible field value for the aws_db_instance to false.

References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_CommonTasks.BackupRestore.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/rds_cluster_instance#publicly_accessible
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance#publicly_accessible