Ensure VPC is enabled for AWS Redshift Cluster

Custom VPC is not defined, default VPC will be used. This may lead to unauthorized network related activities.

New Redshift clusters are created with a default VPC configured. To configure a new VPC, or move the Redshift cluster to another existing VPC, see the AWS documentation.

In Terraform -

1. In the aws_redshift_cluster resource, add the necessary VPC IDs to the vpc_security_group_ids list.

References:
https://docs.aws.amazon.com/redshift/latest/mgmt/managing-cluster-cross-vpc.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/redshift_cluster#vpc_security_group_ids