Covers "Backwards compatibility for installation" from the epic.

The desire here is that scripts, demos, etc people write to install Knative Serving for one released version continue to work in the next version. This means we treat our Operator CR (KnativeServing) as part of our supported API and don't change how we integrate with Service Mesh in a way that breaks for users. Both of these specific items are things that have or will change before GA and perhaps even after GA. We need tests to ensure we can makes these changes in a backwards-compatible way.