Move rancher/hardened-kubernetes build to rancher/image-build-kubernetes

[luthermonson]

Bring https://github.com/rancher/image-build-kubernetes back to life and up-to-date with the current build scripts, mostly cribbed from image-build-kube-proxy; ensure that it also builds the kube-proxy binary.
History: we stopped using it when we decided to build most of the Kubernetes stuff in RKE2, and kube-proxy in the image-build-kube-proxy repo.

#1190

[luthermonson]

rancher/image-build-kubernetes#5
rancher/image-build-kubernetes#6
rancher/image-build-kubernetes#7
rancher/image-build-kubernetes#8
rancher/image-build-kubernetes#9
rancher/image-build-kubernetes#10

[rancher-max]

Validated using master branch commit e889d2e273bd88b8ed273207334f394fac0a6a11

• kube-proxy pod uses correct image: index.docker.io/rancher/hardened-kubernetes:v1.21.3-rke2r1-build20210721
• k8s version shows correctly and rke2 version specifies correctly still as well:

$ kubectl get nodes
NAME               STATUS   ROLES                       AGE   VERSION
ip-172-31-30-224   Ready    control-plane,etcd,master   73m   v1.21.3+rke2r1

$ kubectl version
Client Version: version.Info{Major:"1", Minor:"21", GitVersion:"v1.21.3+rke2r1", GitCommit:"ca643a4d1f7bfe34773c74f79527be4afd95bf39", GitTreeState:"clean", BuildDate:"2021-07-21T16:49:24Z", GoVersion:"go1.16.6b7", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"21", GitVersion:"v1.21.3+rke2r1", GitCommit:"ca643a4d1f7bfe34773c74f79527be4afd95bf39", GitTreeState:"clean", BuildDate:"2021-07-21T16:49:24Z", GoVersion:"go1.16.6b7", Compiler:"gc", Platform:"linux/amd64"}

$ rke2 --version
rke2 version v1.21.3-dev+e889d2e2 (e889d2e273bd88b8ed273207334f394fac0a6a11)
go version go1.16.6b7