Remove annotations and added ingressClassName spec (#1576)

Tested with K8S version 1.26.6 on AKS, without ingressClassName spec pointing to IngressClass the AGIC does not seem to configure anything on ApplicationGateway. 

The default IngressClass is azure-application-gateway from kubectl get IngressClass command

The annotations are removed otherwise ingressClassName spec cannot be used.

Author: chianw

docs/examples/aspnetapp.yaml

@@ -32,9 +32,10 @@ apiVersion: networking.k8s.io/v1
 kind: Ingress
 metadata:
   name: aspnetapp
-  annotations:
-    kubernetes.io/ingress.class: azure/application-gateway
+  # annotations:
+  #   kubernetes.io/ingress.class: azure/application-gateway
 spec:
+  ingressClassName: azure-application-gateway
   rules:
   - http:
       paths: