Increase the minimum supported Kubernetes version to v1.19

Since Kubernetes v1.19, some APIs are deprecated. Add deprecated comment
to those APIs and update documents.

Reference: https://kubernetes.io/docs/reference/using-api/deprecation-guide/

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

@@ -24,6 +24,7 @@ import (
 
 	"github.com/munnerz/goautoneg"
 	v1 "k8s.io/apiextensions-apiserver/pkg/apis/apiextensions/v1"
+	// Deprecated: apiextensions/v1beta1 is deprecated, use apiextensions/v1 instead
 	"k8s.io/apiextensions-apiserver/pkg/apis/apiextensions/v1beta1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"