Decomission porche and serve artifacts.k8s.io from Fastly

[upodroid]

We are spending a fair bit of money serving artifacts.k8s.io from GCS. We have plenty of unused bandwidth from Fastly, so we should decommission porche and serve all the traffic directly from Fastly.

@justinsb @hakman @kubernetes/sig-k8s-infra-leads

[ameukam]

Probably switch to a private bucket+Fastly service before decommission porche.