Federation deployment does not need router mode sni-dnat

In our documentation we create ingress and egress gateways for federated traffic with "routerMode: sni-dnat", but we don't use AUTO_PASSTHROUGH mode to route traffic, between federated meshes, so it never was needed. What's more, environment variable "ISTIO_META_ROUTER_MODE" is ignored since Istio 1.11, look at this PR: https://github.com/istio/istio/pull/33129.

We should remove this setting from the documentation, samples and tests.

Update:
We use AUTO_PASSTHROUGH when Mesh.Spec.Security.AllowDirectInbound is set in ServiceMeshPeer. It will not work as a comment says, but it's better to not change it in maistra-2.1.