Support HTTP/2 in Let's Encrypt (#16371)

Modify the tlsConfig.NextProtos for Let's Encrypt and built-in HTTPS server in order to support HTTP/2.

Co-authored-by: 6543 <6543@obermui.de>

cmd/web_letsencrypt.go

@@ -54,6 +54,7 @@ func runLetsEncrypt(listenAddr, domain, directory, email string, m http.Handler)
 	}
 
 	tlsConfig := magic.TLSConfig()
+	tlsConfig.NextProtos = append(tlsConfig.NextProtos, "h2")
 
 	if enableHTTPChallenge {
 		go func() {