Update publicNetworkAccess to remove readOnly property as this is mut…

…able (#29013)

Co-authored-by: Akshit Goyal <[email protected]>
Co-authored-by: Roopesh Manda <[email protected]>

specification/monitor/resource-manager/Microsoft.Monitor/stable/2023-04-03/monitoringAccounts_API.json

@@ -470,13 +470,14 @@
             "Disabled"
           ],
           "type": "string",
-          "readOnly": true,
           "x-ms-enum": {
             "name": "PublicNetworkAccess",
             "modelAsString": true
           },
           "x-ms-mutability": [
-            "read"
+            "create",
+            "read",
+            "update"
           ]
         }
       }