Update aws-node from 1.12.6 to 1.18.1 (eksctl-io#7756)

* Update aws-node from 1.12.6 to 1.18.1

1.18.1 is recommended for EKS clusters, where its documented that "For
all Kubernetes releases, we recommend installing the latest VPC CNI
release." as read at https://github.com/aws/amazon-vpc-cni-k8s?tab=readme-ov-file#recommended-version.

The latest available addon for various k8s minor versions are listed at
https://docs.aws.amazon.com/eks/latest/userguide/managing-vpc-cni.html#updating-vpc-cni-add-on,
and it currently sais 1.18.1 for k8s 1.23 to 1.29.

* Update tests for aws-node 1.18.1

* Reduce complexity of aws-node test