Retirement of kubernetes-incubator

[Christoph Blecker]

This has been a *long time coming*[1], but as of today, kubernetes-incubator is officially retired.

The incubator process[2] was created to allow for our quickly expanding ecosystem to have a place to build and test against early Kubernetes. However, now our SIG subproject systems and processes have matured and we have finally been able to find proper homes for all of the original incubator projects.

Thank you to everyone that helped out in this process, and to all the original kubernetes-incubator project maintainers.

Cheers,

Christoph

[1]: https://github.com/kubernetes/community/issues/1922

[2]: https://git.k8s.io/community/incubator.md