Knative graduation proposal

[aliok]

On behalf of the Knative Steering Committee, I would like to propose Knative to become a graduated CNCF project.

As you can see in the proposal document in this PR, we believe Knative is ready to graduate.

A TAG CS governance review request was created here: https://github.com/cncf/tag-contributor-strategy/issues/578 (I believe this is not a MUST for graduation yet, as I saw the Istio example).

[angellk]

@amye could you please label this as TAG Runtime and TAG App Delivery?

[amye]

(Graduation proposals don't get labeled with TAGs but this is is fine)