chore: add enableMultipleStandardLoadBalancers to loadBalancerProfile (#15579)

Author: nilo19

specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2021-08-01/managedClusters.json

@@ -3184,6 +3184,10 @@
           "minimum": 4,
           "description": "Desired outbound flow idle timeout in minutes. Allowed values are in the range of 4 to 120 (inclusive). The default value is 30 minutes.",
           "default": 30
+        },
+        "enableMultipleStandardLoadBalancers": {
+          "type": "boolean",
+          "description": "Enable multiple standard load balancers per AKS cluster or not."
         }
       },
       "description": "Profile of the managed cluster load balancer."