fixed core 3.1 startup and verified it still works with 2.1 and net461

Source/MQTTnet.AspnetCore/Extensions/ConnectionRouteBuilderExtensions.cs

@@ -0,0 +1,20 @@
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Connections;
+using System;
+
+namespace MQTTnet.AspNetCore
+{
+    public static class ConnectionRouteBuilderExtensions
+    {
+#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+        [Obsolete("This class is obsolete and will be removed in a future version.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
+        public static void MapMqtt(this ConnectionsRouteBuilder connection, PathString path)
+        {
+            connection.MapConnectionHandler<MqttConnectionHandler>(path, options =>
+            {
+                options.WebSockets.SubProtocolSelector = MqttSubProtocolSelector.SelectSubProtocol;
+            });
+        }
+    }
+}