coreos-base/flatcar-eks: update supported kubernetes versions for cgroupv2

We are switching flatcar to cgroupv2 which is support by docker 20.10 and
kubernetes 1.19. This requires setting the systemd cgroup driver in the kubelet
config.

Due to the unified cgroup hierarchy, kubernetes <1.19 will not work so
remove all older versions.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

sdk_container/src/third_party/coreos-overlay/coreos-base/flatcar-eks/files/docker-daemon.json

@@ -1,5 +1,8 @@
 {
   "bridge": "none",
+  "exec-opts": [
+    "native.cgroupdriver=systemd"
+  ],
   "log-driver": "json-file",
   "log-opts": {
     "max-size": "10m",

sdk_container/src/third_party/coreos-overlay/coreos-base/flatcar-eks/files/download-kubelet.sh

@@ -20,7 +20,17 @@ if [[ -z "${CLUSTER_VERSION}" ]]; then
 fi
 
 # Select the right path depending on the Kubernetes version.
+# https://github.com/awslabs/amazon-eks-ami/blob/master/Makefile
 case $CLUSTER_VERSION in
+  1.21)
+    S3_PATH="1.21.2/2021-07-05"
+    ;;
+  1.20)
+    S3_PATH="1.20.4/2021-04-12"
+    ;;
+  1.19)
+    S3_PATH="1.19.6/2021-01-05"
+    ;;
   1.18)
     S3_PATH="1.18.9/2020-11-02"
     ;;

sdk_container/src/third_party/coreos-overlay/coreos-base/flatcar-eks/files/kubelet-config.json

@@ -24,7 +24,7 @@
   "clusterDomain": "cluster.local",
   "hairpinMode": "hairpin-veth",
   "readOnlyPort": 0,
-  "cgroupDriver": "cgroupfs",
+  "cgroupDriver": "systemd",
   "cgroupRoot": "/",
   "featureGates": {
     "RotateKubeletServerCertificate": true