[NotificationHubs] Namespace properties not marked as readonly

In NotificationHubs, several properties of Namespace should be marked as read-only, for instance `createdAt` and `updatedAt`: https://github.com/Azure/azure-rest-api-specs/blob/7a2cc29033fe4027ef421267f1684efbd0d40a93/specification/notificationhubs/resource-manager/Microsoft.NotificationHubs/stable/2017-04-01/notificationhubs.json#L1642-L1651

Open specification/notificationhubs/resource-manager/Microsoft.NotificationHubs/stable/2017-04-01/notificationhubs.json at lines 1642-1651 and inspect the Namespace properties, including createdAt and updatedAt. Identify the applicable properties that should be read-only. Done means those Namespace properties are marked read-only in the API definition.

Written by the indexing model from the issue text.