cmd/derper: disable http2

DERP doesn't support HTTP/2. If an HTTP/2 proxy was placed in front of
a DERP server requests would fail because the connection would
be initialized with HTTP/2, which the DERP client doesn't support.

Signed-off-by: Kyle Carberry <kyle@carberry.com>

cmd/derper/cert.go

@@ -81,7 +81,7 @@ func (m *manualCertManager) TLSConfig() *tls.Config {
 	return &tls.Config{
 		Certificates: nil,
 		NextProtos: []string{
-			"h2", "http/1.1", // enable HTTP/2
+			"http/1.1",
 		},
 		GetCertificate: m.getCertificate,
 	}