fix: update kube version requirement to 1.21 (#11275)

The controller depends on the v1 version of EndpointSlice, but the discovery.k8s.io/v1 API was first introduced in Kubernetes version 1.21.

Author: NierYYDS

charts/ingress-nginx/Chart.yaml

@@ -11,7 +11,7 @@ icon: https://upload.wikimedia.org/wikipedia/commons/thumb/c/c5/Nginx_logo.svg/5
 keywords:
   - ingress
   - nginx
-kubeVersion: '>=1.20.0-0'
+kubeVersion: '>=1.21.0-0'
 maintainers:
   - name: Gacko
   - name: rikatz

charts/ingress-nginx/README.md

@@ -10,7 +10,7 @@ This chart bootstraps an ingress-nginx deployment on a [Kubernetes](http://kuber
 
 ## Requirements
 
-Kubernetes: `>=1.20.0-0`
+Kubernetes: `>=1.21.0-0`
 
 ## Get Repo Info