Consider using alternative source for kubeval schemas

[spencergilbert]

instrumenta/kubernetes-json-schema is noticeably out of date at this point, with the last commit to master landing in April of 2020, this puts the latest provided Kubernetes version at 1.18.1.

As an alternative there is yannh/kubernetes-json-schema which is self-updating and currently up-to-date.

It's a trivial change to update the schema source (see: here).

This would allow for the Helm charts to be evaluated against more, and more recent versions of Kubernetes.

[wyardley]

I added that back in #664 FWIW, so this original issue should be resolved.

Also have just opened #1199 which proposes moving to kubeconform entirely.