Update Jsonnet lib to work with latest Tanka version

[fionaliao]

The default k8s library used by Tanka was changed in grafana/tanka#563. This meant the k8s instructions didn't work with the latest version of Tanka (v0.20); I was seeing an error like:

$ tk show environments/default/
Error: evaluating jsonnet: RUNTIME ERROR: Attempt to use super when there is no super class.
        /.../crng-tanka-check/vendor/github.com/jsonnet-libs/k8s-libsonnet/1.20/_custom/apps.libsonnet:35:7-12 function <anonymous>
        /.../crng-tanka-check/vendor/ksonnet-util/grafana.libsonnet:45:9-57    function <anonymous>
        /.../crng-tanka-check/vendor/carbon-relay-ng/crng.libsonnet:62:5-94    object <anonymous>
        Field "carbon_relay_ng_deployment"
        Field "crng"
        During manifestation

This PR updates the carbon-relay-ng jsonnet lib so it'll work with the updated k8s library, and also makes a small update to the k8s doc.

Testing

Followed the k8s instructions to set up the Tanka environment, ran tk show environments/default, which worked after I'd made changes to the jsonnet library.