Bump k8s libs to 1.19

[nak3]

/area API
/area test-and-release
/kind feature

This is similar request to #1280, but for v1.19.

For example, service-apis repo uses client-go v0.19.2 and so we hit a dependency error when using the latest service-apis in knative/networking repo.

/cc @dprotaso @mattmoor

[nak3]

Just in case, this is not urgent request for me at this moment. I am testing serving-apis and could manage it on my local and will take some time until send/merge the PR in networking repo.

[mattmoor]

I'd have to check the schedule, but I don't think we can do this until K8s 1.20 cuts, which is after our 0.19 cuts IIUC.

Let's discuss in a little over a week?

[dprotaso]

Related: knative/community#245 (comment)