[ReleasePR containerinstance] {ContainerInstance} fixes Azure/azure-rest-api-specs#19195 (#2439)

Create to sync Azure/azure-rest-api-specs#19532
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...azure-sdk:sdkAuto/containerinstance?expand=1)

Author: msftbot[bot]

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

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