### 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
#endif
	Engine::get_singleton()->set_frame_delay(frame_delay);
	message_queue = memnew(MessageQueue);
	engine->startup_benchmark_end_measure(); 
	if (p_second_phase) {
		return setup2();
	}
	return OK;
error:
	text_driver = "";
	display_driver = "";
	audio_driver = "";
	tablet_driver = "";
	Engine::get_singleton()->set_write_movie_path(String());
	project_path = "";
	args.clear();
	main_args.clear();
	if (show_help) {
		print_help(execpath);
	}
	EngineDebugger::deinitialize();
	if (performance) {
		memdelete(performance);
	}
	if (input_map) {
		memdelete(input_map);
	}
	if (time_singleton) {