### 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
return false;
		}
		if (script_res->can_instantiate()) {
			StringName instance_type = script_res->get_instance_base_type();
			Object *obj = ClassDB::instantiate(instance_type);
			MainLoop *script_loop = Object::cast_to<MainLoop>(obj);
			if (!script_loop) {
				if (obj) {
					memdelete(obj);
				}
				OS::get_singleton()->alert(vformat("Can't load the script \"%s\" as it doesn't inherit from SceneTree or MainLoop.", script));
				ERR_FAIL_V_MSG(false, vformat("Can't load the script \"%s\" as it doesn't inherit from SceneTree or MainLoop.", script));
			}
			script_loop->set_initialize_script(script_res);
			main_loop = script_loop;
		} else {
			return false;
		}
	} else { /
		if (!editor && !ClassDB::class_exists(main_loop_type) && ScriptServer::is_global_class(main_loop_type)) {
			String script_path = ScriptServer::get_global_class_path(main_loop_type);
			Ref<Script> script_res = ResourceLoader::load(script_path);
			if (script_res.is_null()) {
				OS::get_singleton()->alert("Error: Could not load MainLoop script type: " + main_loop_type);
				ERR_FAIL_V_MSG(false, vformat("Could not load global class %s.", main_loop_type));
			}
			StringName script_base = script_res->get_instance_base_type();
			Object *obj = ClassDB::instantiate(script_base);
			MainLoop *script_loop = Object::cast_to<MainLoop>(obj);
			if (!script_loop) {