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.

dot.go

@@ -64,7 +64,8 @@ func newDOTClientPool(uc *UpstreamConfig, addrs []string) *dotConnPool {
 	dialer := newDialer(net.JoinHostPort(controldPublicDns, "53"))
 
 	tlsConfig := &tls.Config{
-		RootCAs: uc.certPool,
+		RootCAs:    uc.certPool,
+		MinVersion: tls.VersionTLS12,
 	}
 
 	if uc.BootstrapIP != "" {