### 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 (debug_collisions) {
			sml->set_debug_collisions_hint(true);
		}
		if (debug_paths) {
			sml->set_debug_paths_hint(true);
		}
		if (debug_navigation) {
			sml->set_debug_navigation_hint(true);
			NavigationServer3D::get_singleton()->set_active(true);
			NavigationServer3D::get_singleton()->set_debug_enabled(true);
		}
#endif
		bool embed_subwindows = GLOBAL_DEF("display/window/subwindows/embed_subwindows", true);
		if (single_window || (!project_manager && !editor && embed_subwindows) || !DisplayServer::get_singleton()->has_feature(DisplayServer::Feature::FEATURE_SUBWINDOWS)) {
			sml->get_root()->set_embedding_subwindows(true);
		}
		ResourceLoader::add_custom_loaders();
		ResourceSaver::add_custom_savers();
		if (!project_manager && !editor) { /
			if (!game_path.is_empty() || !script.is_empty()) {
				/
				Engine::get_singleton()->startup_benchmark_begin_measure("load_autoloads");
				HashMap<StringName, ProjectSettings::AutoloadInfo> autoloads = ProjectSettings::get_singleton()->get_autoload_list();
				/
				for (const KeyValue<StringName, ProjectSettings::AutoloadInfo> &E : autoloads) {
					const ProjectSettings::AutoloadInfo &info = E.value;
					if (info.is_singleton) {
						for (int i = 0; i < ScriptServer::get_language_count(); i++) {
							ScriptServer::get_language(i)->add_global_constant(info.name, Variant());
						}