fix(inventory): Add the `prune` field to `Kustomization`.

fix(inventory): Add the prune field to Kustomization.

.spec.prune is a required boolean field to enable/disable garbage collection for a Kustomization.

Fixes:

Kustomization/runway-provisioner/example-service-gke dry-run failed (Invalid): Kustomization.kustomize.toolkit.fluxcd.io "example-service-gke" is invalid: spec.prune: Required value

Issue: team#288 (closed)