Generate self-sign cert with SHA256

[anattapol]

Summary

When apply a pod, I faced this error.

$ kubectl -n $ns apply -f dev/manifests/pods/bad-name.pod.yaml
Error from server (InternalError): error when creating "dev/manifests/pods/bad-name.pod.yaml": Internal error occurred: failed calling webhook "simple-kubernetes-webhook.acme.com": failed to call webhook: Post "https://simple-kubernetes-webhook.default.svc:443/mutate-pods?timeout=2s": x509: certificate signed by unknown authority (possibly because of "x509: cannot verify signature: insecure algorithm SHA1-RSA (temporarily override with GODEBUG=x509sha1=1)" while trying to verify candidate authority certificate "simple-kubernetes-webhook")

The cert used the insecure algorithm SHA1-RSA. That will be blocked by crypto/x509. https://github.com/golang/go/issues/41682

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