[dotnet] Support IPv6 to find free tcp port via DualMode

dotnet/src/webdriver/Internal/PortUtilities.cs

@@ -33,23 +33,10 @@ public static class PortUtilities
     /// <returns>A random, free port to be listened on.</returns>
     public static int FindFreePort()
     {
-        // Locate a free port on the local machine by binding a socket to
-        // an IPEndPoint using IPAddress.Any and port 0. The socket will
-        // select a free port.
-        int listeningPort = 0;
-        Socket portSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
-        try
-        {
-            IPEndPoint socketEndPoint = new IPEndPoint(IPAddress.Any, 0);
-            portSocket.Bind(socketEndPoint);
-            socketEndPoint = (IPEndPoint)portSocket.LocalEndPoint!;
-            listeningPort = socketEndPoint.Port;
-        }
-        finally
-        {
-            portSocket.Close();
-        }
+        using var socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
+        socket.DualMode = true;
+        socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
+        return (socket.LocalEndPoint as IPEndPoint)!.Port;
 
-        return listeningPort;
     }
 }