all: explicit TLS MinVersion in tls.Config

Go's default is already TLS 1.2+ (since Go 1.18), but making this
explicit satisfies RFC 7858/9250 recommendations and makes the security
intent clear for auditors.

doq_test.go

@@ -99,6 +99,7 @@ func newTestQUICServer(t *testing.T) *testQUICServer {
 	tlsConfig := &tls.Config{
 		Certificates: []tls.Certificate{testCert.tlsCert},
 		NextProtos:   []string{"doq"},
+		MinVersion:   tls.VersionTLS12,
 	}
 
 	// Create QUIC listener