Ping kubernetes version to 8.0
Ping kubernetes version to 8.0

9.0 alpha has breaking changes, and our version selector
was including 9.0alpha.

Breaking changes listed here:

https://github.com/kubernetes-client/python/blob/master/CHANGELOG.md#v900a1
yuvipanda/jupyterhub-kubernetes-spawner,jupyterhub/kubespawner