apiserver: add localhost to alternateDNS for IPv6

apiserver adds localhost to the alternateDNS field
if the bind address is 0.0.0.0.
This PR considers the IPv6 unspecified address too.

staging/src/k8s.io/apiserver/pkg/server/options/serving.go

@@ -287,8 +287,7 @@ func (s *SecureServingOptions) MaybeDefaultWithSelfSignedCerts(publicAddress str
 
 	if !canReadCertAndKey {
 		// add either the bind address or localhost to the valid alternates
-		bindIP := s.BindAddress.String()
-		if bindIP == "0.0.0.0" {
+		if s.BindAddress.IsUnspecified() {
 			alternateDNS = append(alternateDNS, "localhost")
 		} else {
 			alternateIPs = append(alternateIPs, s.BindAddress)