### Godot version

master

### System information

Ubuntu

### Issue description

I think one of the recent changes wrapped the project converter in `#ifndef DISABLE_DEPRECATED` but missed the coresponding code in `main.cpp` (and potentially other places).
```
main/main.cpp: In static member function 'static bool Main::start()':
main/main.cpp:2551:33: error: 'ProjectConverter3To4' was not declared in this scope
 2551 |                 int exit_code = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).convert();
      |                                 ^~~~~~~~~~~~~~~~~~~~
main/main.cpp:2556:33: error: 'ProjectConverter3To4' was not declared in this scope
 2556 |                 int exit_code = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).validate_conversion();
```

### Steps to reproduce

for instance, try to compile with `deprecated = "no"` in your `custom.py`.

### Minimal reproduction project

n/a
register_driver_types();
	initialize_modules(MODULE_INITIALIZATION_LEVEL_SCENE);
	GDExtensionManager::get_singleton()->initialize_extensions(GDExtension::INITIALIZATION_LEVEL_SCENE);
#ifdef TOOLS_ENABLED
	ClassDB::set_current_api(ClassDB::API_EDITOR);
	register_editor_types();
	initialize_modules(MODULE_INITIALIZATION_LEVEL_EDITOR);
	GDExtensionManager::get_singleton()->initialize_extensions(GDExtension::INITIALIZATION_LEVEL_EDITOR);
	ClassDB::set_current_api(ClassDB::API_CORE);
#endif
	MAIN_PRINT("Main: Load Modules");
	register_platform_apis();
	
	initialize_theme_db();
	register_scene_singletons();
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::STRING, "display/mouse_cursor/custom_image", PROPERTY_HINT_FILE, "*.png,*.webp"), String());
	GLOBAL_DEF_BASIC("display/mouse_cursor/custom_image_hotspot", Vector2());
	GLOBAL_DEF_BASIC("display/mouse_cursor/tooltip_position_offset", Point2(10, 10));
	if (String(GLOBAL_GET("display/mouse_cursor/custom_image")) != String()) {
		Ref<Texture2D> cursor = ResourceLoader::load(
				GLOBAL_GET("display/mouse_cursor/custom_image"));
		if (cursor.is_valid()) {
			Vector2 hotspot = GLOBAL_GET("display/mouse_cursor/custom_image_hotspot");
			Input::get_singleton()->set_custom_mouse_cursor(cursor, Input::CURSOR_ARROW, hotspot);
		}
	}
	camera_server = CameraServer::create();
	MAIN_PRINT("Main: Load Physics");
	initialize_physics();
	initialize_navigation_server();