fix(ksonnet-util): don't dictate upstream library

In 91bbd26, kausal was fixed to
`ksonnet-lib/ksonnet.beta.4`, which is problematic because:

- it prevents use of other API compatible libraries, notably
`jsonnet-libs/k8s` and `kube-jsonnet/k`
- it forces users to use outdated, unmaintained, archived software
- it bypasses a Tanka measure for this exact case, namely the "alias
import" `k.libsonnet`, which has been explicitely set up for importing
the Kubernetes library in the correct version for the users cluster.

Given that, I think we should revert most of these changes, keeping the
actual fix of that PR (a function signature backport).

I don't think this will have any impact on our internal config repo, as
it already uses `ksonnet.beta.4` through the alias import, so
this *should* be a no-op.

ksonnet-util/kausal.libsonnet

@@ -1,5 +1,5 @@
 // Override defaults paramters for objects in the ksonnet libs here.
-local k = import 'ksonnet.beta.4/k.libsonnet';
+local k = import 'k.libsonnet';
 
 k {
   _config+:: {