### 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
if (!p_force) {
		ERR_FAIL_COND(!_start_success);
	}
	for (int i = 0; i < TextServerManager::get_singleton()->get_interface_count(); i++) {
		TextServerManager::get_singleton()->get_interface(i)->cleanup();
	}
	if (movie_writer) {
		movie_writer->end();
	}
	ResourceLoader::remove_custom_loaders();
	ResourceSaver::remove_custom_savers();
	/
	message_queue->flush();
	OS::get_singleton()->delete_main_loop();
	OS::get_singleton()->_cmdline.clear();
	OS::get_singleton()->_user_args.clear();
	OS::get_singleton()->_execpath = "";
	OS::get_singleton()->_local_clipboard = "";
	ResourceLoader::clear_translation_remaps();
	ResourceLoader::clear_path_remaps();
	ResourceLoader::clear_thread_load_tasks();
	ScriptServer::finish_languages();
	/
	RenderingServer::get_singleton()->sync();