Kubernetes provider for 1.20 upgrade

[vijay-veeranki]

Using the latest Kubernetes provider having issues

terraform-aws-modules/terraform-aws-eks#1280 (comment)

The suggestion above is to upgrade to terraform 1.0, but that is a separate epic to upgrade to terraform 1.0

The current Kubernetes provider version is 1.11, but that is not working for the 1.20 upgrade, find the right version for the 1.20 upgrade, which runs without any issues.

[vijay-veeranki]

Applied in this PR

ministryofjustice/cloud-platform-infrastructure#1433

Upgrade k8's provider to version = "~> 2.6.1", this is a pre-requisite for K8s 1.20 upgrade.

https://registry.terraform.io/providers/hashicorp/kubernetes/latest/docs/guides/v2-upgrade-guide#changes-in-v200

The load_config_file attribute has been removed.
Support for the KUBECONFIG environment variable has been dropped. (Use KUBE_CONFIG_PATH or KUBE_CONFIG_PATHS instead).
The config_path attribute will no longer default to ~/.kube/config.
If KUBE_CONFIG_PATH is set, it is ignoring below, so we need to unset KUBE_CONFIG_PATH, unless we want to set config_path

  host                   = data.aws_eks_cluster.cluster.endpoint
  cluster_ca_certificate = base64decode(data.aws_eks_cluster.cluster.certificate_authority.0.data)
  token                  = data.aws_eks_cluster_auth.cluster.token

Used exec as to avoid token expire during long-running applies.