[AKS] properly deprecate "az aks create --enable-rbac" argument

[mboersma]

Improves on the hand-rolled deprecation notice in #6648 by using the framework added in #6472.

$ az aks create -h

Command
    az aks create : Create a new managed Kubernetes cluster.

Arguments
...
    --enable-addons -a             : Enable the Kubernetes addons in a comma-separated list.
        These addons are available:
            http_application_routing - configure ingress with automatic public DNS name creation.
            monitoring - turn on Log Analytics monitoring. Uses the Log Analytics Default Workspace
        if it exists, else creates one. Specify "--workspace-resource-id" to use an existing
        workspace.
    --enable-rbac -r  [Deprecated] : Enable Kubernetes Role-Based Access Control. Default:
                                     enabled.
        Argument 'enable_rbac' has been deprecated and will be removed in a future release. Use
        '--disable-rbac' instead.
    --generate-ssh-keys            : Generate SSH public and private key files if missing.
...

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