This message was deleted.

It's not. It's generated internally by the apiserver when the --cert-dir option is added to the kube-apiserver args. K3s doesn't enable this by default, but rancher does.

So,I should to check the api-server's source code.Right?Beacause I want to generate a certificate with a specified validity period when I installing K3s

You'd need to generate your own cert, put it in a location accessible to k3s, and then add the correct apiserver args to tell the apiserver to use it.