[k3d-dev.sh] Keep K3S image and Kubectl versions in lock step with each other.

Currently today, with the k3d-dev.sh script K3S images are pinned to a specific version (or overridden with !2648 (merged)). The kubectl version installed is always the latest tag. Given that we could override now, we should avoid version mismatch by keeping kubectl and the K8S version in lock step with each other.