let the OS choose the supported TLS ptotocols when connecting to a MQTT broker (#1271)

Source/MQTTnet/Client/Options/MqttClientOptionsBuilderTlsParameters.cs

@@ -19,11 +19,7 @@ namespace MQTTnet.Client.Options
 
         public Func<MqttClientCertificateValidationCallbackContext, bool> CertificateValidationHandler { get; set; }
 
-#if NETCOREAPP3_1 || NET5_0
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls13;
-#else
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
-#endif
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.None;
 
 #if WINDOWS_UWP
         public IEnumerable<IEnumerable<byte>> Certificates { get; set; }

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Net.Security;
 using System.Security.Authentication;
@@ -26,11 +26,7 @@ namespace MQTTnet.Client.Options
         public List<SslApplicationProtocol> ApplicationProtocols { get; set; }
 #endif
 
-#if NETCOREAPP3_1 || NET5_0
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls13;
-#else
-        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
-#endif
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.None;
 
         [Obsolete("This property will be removed soon. Use CertificateValidationHandler instead.")]
         public Func<X509Certificate, X509Chain, SslPolicyErrors, IMqttClientOptions, bool> CertificateValidationCallback { get; set; }