`--enable-bootstrap-token-auth=true` should be set when using v1.8.0-beta.0 or higher

[luxas]

Thanks to kubernetes/kubernetes#51198, we now have a beta flag for enabling the BootstrapTokenAuthenticator. We should use the old flag (--experimental-bootstrap-token-auth) for versions below v1.8.0-beta.0, but the new one (--enable-bootstrap-token-auth) for all versions equal to and above.

I'm sending a PR for this

cc @mattmoyer