CodeGen from PR 19532 in Azure/azure-rest-api-specs

{ContainerInstance} fixes Azure/azure-rest-api-specs#19195 (#19532)

fixes Azure/azure-rest-api-specs#19195

The below articles talks about the username field should not be required if identity is present.
https://docs.microsoft.com/en-us/azure/container-instances/using-azure-container-registry-mi#deploy-using-an-azure-resource-manager-arm-template

Author: SDKAuto

schemas/2021-10-01/Microsoft.ContainerInstance.json

@@ -859,8 +859,7 @@
         }
       },
       "required": [
-        "server",
-        "username"
+        "server"
       ],
       "description": "Image registry credential."
     },