### 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
ERR_FAIL_COND(!_start_success);
	for (int i = 0; i < TextServerManager::get_singleton()->get_interface_count(); i++) {
		TextServerManager::get_singleton()->get_interface(i)->cleanup();
	}
	EngineDebugger::deinitialize();
	ResourceLoader::remove_custom_loaders();
	ResourceSaver::remove_custom_savers();
#ifdef TOOLS_ENABLED
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_EDITOR);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_EDITOR);
	unregister_editor_types();
#endif
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_SCENE);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_SCENE);
	unregister_platform_apis();
	unregister_driver_types();
	unregister_scene_types();
	finalize_theme_db();
	GDExtensionManager::get_singleton()->deinitialize_extensions(GDExtension::INITIALIZATION_LEVEL_SERVERS);
	uninitialize_modules(MODULE_INITIALIZATION_LEVEL_SERVERS);
	unregister_server_types();