Use h2c from x/net to handle h2c requests

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

vendor/golang.org/x/net/ipv6/helper.go

@@ -7,14 +7,16 @@ package ipv6
 import (
 	"errors"
 	"net"
+	"runtime"
 )
 
 var (
+	errInvalidConn     = errors.New("invalid connection")
 	errMissingAddress  = errors.New("missing address")
 	errHeaderTooShort  = errors.New("header too short")
 	errInvalidConnType = errors.New("invalid conn type")
-	errOpNoSupport     = errors.New("operation not supported")
 	errNoSuchInterface = errors.New("no such interface")
+	errNotImplemented  = errors.New("not implemented on " + runtime.GOOS + "/" + runtime.GOARCH)
 )
 
 func boolint(b bool) int {