Increase the minimum supported Kubernetes version to v1.19 (#6089)

Starting in v1.19, a CustomResourceDefinition can indicate a particular
version of the resource it defines is deprecated. See:
https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/#version-deprecation

This will be useful in the future when we want to deprecated CRD versions.

Signed-off-by: hjiajing <hjiajing@vmware.com>

README.md

@@ -27,7 +27,7 @@ Network Policies in a very efficient manner.
 
 ## Prerequisites
 
-Antrea has been tested with Kubernetes clusters running version 1.16 or later.
+Antrea has been tested with Kubernetes clusters running version 1.19 or later.
 
 * `NodeIPAMController` must be enabled in the Kubernetes cluster.\
   When deploying a cluster with kubeadm the `--pod-network-cidr <cidr>`

pkg/apiserver/handlers/webhook/convert_crd.go

@@ -45,6 +45,7 @@ func statusSucceed() metav1.Status {
 
 // doConversionV1beta1 converts the requested objects in the v1beta1 ConversionRequest using the given conversion function and
 // returns a conversion response. Failures are reported with the Reason in the conversion response.
+// Deprecated: apiextensions/v1beta1 is deprecated, use apiextensions/v1 instead
 func doConversionV1beta1(convertRequest *v1beta1.ConversionRequest, convert convertFunc) *v1beta1.ConversionResponse {
 	var convertedObjects []runtime.RawExtension
 	for _, obj := range convertRequest.Objects {