### 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_server_singletons();
	/
	ScriptServer::init_languages();
	audio_server->load_default_bus_layout();
	if (use_debug_profiler && EngineDebugger::is_active()) {
		/
		/
		EngineDebugger::get_singleton()->profiler_enable("scripts", true);
	}
	if (!project_manager) {
		/
		/
		/
		/
		rendering_server->global_shader_parameters_load_settings(!editor);
	}
	_start_success = true;
	ClassDB::set_current_api(ClassDB::API_NONE); /
	print_verbose("CORE API HASH: " + uitos(ClassDB::get_api_hash(ClassDB::API_CORE)));
	print_verbose("EDITOR API HASH: " + uitos(ClassDB::get_api_hash(ClassDB::API_EDITOR)));
	MAIN_PRINT("Main: Done");
	engine->startup_benchmark_end_measure(); /
	return OK;
}
String Main::get_rendering_driver_name() {
	return rendering_driver;
}
/
static MainTimerSync main_timer_sync;
bool Main::start() {