Upgrade Kubernetes Provider

[vijay-veeranki]

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.