http2: disable TLS 1.3 in failing test temporarily(?)

Updates golang/go#28762

Change-Id: If73b292f28e553646431af995942169ce58d43f5
Reviewed-on: https://go-review.googlesource.com/c/149357
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

http2/server_test.go

@@ -2416,6 +2416,7 @@ func testRejectTLS(t *testing.T, max uint16) {
 
 func TestServer_Rejects_TLSBadCipher(t *testing.T) {
 	st := newServerTester(t, nil, func(c *tls.Config) {
+		c.MaxVersion = tls.VersionTLS12 // workaround for golang.org/issue/28762
 		// Only list bad ones:
 		c.CipherSuites = []uint16{
 			tls.TLS_RSA_WITH_RC4_128_SHA,