fixed .net 5 build

Source/MQTTnet.AspnetCore/Extensions/ConnectionRouteBuilderExtensions.cs

@@ -12,6 +12,7 @@ namespace MQTTnet.AspNetCore.Extensions
 #if NETCOREAPP3_1
         [Obsolete("This class is obsolete and will be removed in a future version. The recommended alternative is to use MapMqtt inside Microsoft.AspNetCore.Builder.UseEndpoints(...).")]
 #endif
+#if NETCOREAPP3_1 || NETCOREAPP2_1 || NETSTANDARD
         public static void MapMqtt(this ConnectionsRouteBuilder connection, PathString path)
         {
             connection.MapConnectionHandler<MqttConnectionHandler>(path, options =>
@@ -19,5 +20,6 @@ namespace MQTTnet.AspNetCore.Extensions
                 options.WebSockets.SubProtocolSelector = MqttSubProtocolSelector.SelectSubProtocol;
             });
         }
+#endif
     }
 }