Upgrade to .NET Core 6 (Windows Only Target) and fix UI thread deadlock bug

ntfysh_client/Program.cs

@@ -16,6 +16,7 @@ namespace ntfysh_client
         [STAThread]
         static void Main()
         {
+            //Application.SetHighDpiMode(HighDpiMode.SystemAware);
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
             Application.Run(new Form1(NotificationListener));