Single NAT for multiple private subnets as default

[bquintas]

A colleague mentioned that even though both the EKS control plane and EKS worker nodes are deployed in HA, the NAT Gateway is deployed in a single AZ:
https://github.com/aws-ia/terraform-aws-mendix-private-cloud/blob/main/modules/vpc/main.tf

single_nat_gateway = true

I understand some customers would want to keep their costs down in pilots/testing but I suggest changing the default to multi AZ and highlight in the documentation some changes that customers can take to reduce costs (single AZ NAT, change instance type,etc)