Fix Linux shell window default size (#59630)

The previous call set the minimum size, not just the initial size.

packages/flutter_tools/templates/app/linux.tmpl/my_application.cc

@@ -20,9 +20,9 @@ static void my_application_activate(GApplication* application) {
   gtk_header_bar_set_title(header_bar, kFlutterWindowTitle);
   gtk_header_bar_set_show_close_button(header_bar, TRUE);
   gtk_window_set_titlebar(window, GTK_WIDGET(header_bar));
-  gtk_widget_show(GTK_WIDGET(window));
-  gtk_widget_set_size_request(GTK_WIDGET(window), kFlutterWindowWidth,
+  gtk_window_set_default_size(window, kFlutterWindowWidth,
                               kFlutterWindowHeight);
+  gtk_widget_show(GTK_WIDGET(window));
 
   g_autoptr(FlDartProject) project = fl_dart_project_new();