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

Source/MQTTnet.AspnetCore/MqttSubProtocolSelector.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MQTTnet.AspNetCore
+{
+    public static class MqttSubProtocolSelector
+    {
+        public static string SelectSubProtocol(IList<string> requestedSubProtocolValues)
+        {
+            // Order the protocols to also match "mqtt", "mqttv-3.1", "mqttv-3.11" etc.
+            return requestedSubProtocolValues
+                .OrderByDescending(p => p.Length)
+                .FirstOrDefault(p => p.ToLower().StartsWith("mqtt"));
+        }
+    }
+}