X go/neo: Only accept TLS >= 1.2

Tighten up security. NEO/py does something similar:

https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-61-gc1c26894/neo/lib/app.py#L76-78

On CPython 2.7.18:

    In [7]: list((k for k in dir(ssl) if k.startswith("PROTOCOL_TLSv")))
    Out[7]: ['PROTOCOL_TLSv1', 'PROTOCOL_TLSv1_1', 'PROTOCOL_TLSv1_2']

go/neo/util.go

@@ -104,10 +104,9 @@ func tlsForSSL(ca, cert, key string) (_ *tls.Config, err error) {
 		ClientCAs: CA,
 
 		PreferServerCipherSuites: true,
+		MinVersion:               tls.VersionTLS12, // only accept TLS >= 1.2
 	}
 
-	// TODO only accept TLS >= 1.2 ?
-
 	// tls docs say we should parse Certificate[0] into Leaf ourselves
 	leaf, err := x509.ParseCertificate(crt.Certificate[0])
 	if err != nil {