Update AppServicePlan.json with preoperty (#14388)

Adding ElasticScaleEnabled to ASP object!

Author: jocawtho

specification/web/resource-manager/Microsoft.Web/stable/2021-01-01/AppServicePlans.json

@@ -1509,6 +1509,10 @@
               "default": false,
               "type": "boolean"
             },
+            "elasticScaleEnabled": {
+              "description": "ServerFarm supports ElasticScale. Apps in this plan will scale as if the ServerFarm was ElasticPremium sku",
+              "type": "boolean"
+            },
             "maximumElasticWorkerCount": {
               "format": "int32",
               "description": "Maximum number of total workers allowed for this ElasticScaleEnabled App Service Plan",