add IPv6 listen directive to nginx if enable_dual_stack_networks (#8596)

roles/kubernetes/node/templates/loadbalancer/nginx.conf.j2

@@ -20,6 +20,9 @@ stream {
 
   server {
     listen        127.0.0.1:{{ loadbalancer_apiserver_port|default(kube_apiserver_port) }};
+    {% if enable_dual_stack_networks -%}
+    listen        [::]:{{ loadbalancer_apiserver_port|default(kube_apiserver_port) }};
+    {% endif -%}
     proxy_pass    kube_apiserver;
     proxy_timeout 10m;
     proxy_connect_timeout 1s;
@@ -41,6 +44,9 @@ http {
   {% if loadbalancer_apiserver_healthcheck_port is defined -%}
   server {
     listen {{ loadbalancer_apiserver_healthcheck_port }};
+    {% if enable_dual_stack_networks -%}
+    listen [::]:{{ loadbalancer_apiserver_healthcheck_port }};
+    {% endif -%}
     location /healthz {
       access_log off;
       return 200;