### 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 (obj) {
					memdelete(obj);
				}
				OS::get_singleton()->alert("Error: Invalid MainLoop script base type: " + script_base);
				ERR_FAIL_V_MSG(false, vformat("The global class %s does not inherit from SceneTree or MainLoop.", main_loop_type));
			}
			script_loop->set_initialize_script(script_res);
			main_loop = script_loop;
		}
	}
	if (!main_loop && main_loop_type.is_empty()) {
		main_loop_type = "SceneTree";
	}
	if (!main_loop) {
		if (!ClassDB::class_exists(main_loop_type)) {
			OS::get_singleton()->alert("Error: MainLoop type doesn't exist: " + main_loop_type);
			return false;
		} else {
			Object *ml = ClassDB::instantiate(main_loop_type);
			ERR_FAIL_COND_V_MSG(!ml, false, "Can't instance MainLoop type.");
			main_loop = Object::cast_to<MainLoop>(ml);
			if (!main_loop) {
				memdelete(ml);
				ERR_FAIL_V_MSG(false, "Invalid MainLoop type.");
			}
		}
	}
	SceneTree *sml = Object::cast_to<SceneTree>(main_loop);
	if (sml) {
#ifdef DEBUG_ENABLED