### 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
print_line("Loading docs...");
		for (int i = 0; i < _doc_data_class_path_count; i++) {
			/
			String path = _doc_data_class_paths[i].path;
			if (path.is_relative_path()) {
				path = doc_tool_path.path_join(path);
			}
			String name = _doc_data_class_paths[i].name;
			doc_data_classes[name] = path;
			if (!checked_paths.has(path)) {
				checked_paths.insert(path);
				/
				Ref<DirAccess> da = DirAccess::create_for_path(path);
				err = da->make_dir_recursive(path);
				ERR_FAIL_COND_V_MSG(err != OK, false, "Error: Can't create directory: " + path + ": " + itos(err));
				print_line("Loading docs from: " + path);
				err = docsrc.load_classes(path);
				ERR_FAIL_COND_V_MSG(err != OK, false, "Error loading docs from: " + path + ": " + itos(err));
			}
		}
		String index_path = doc_tool_path.path_join("doc/classes");
		/
		Ref<DirAccess> da = DirAccess::create_for_path(index_path);
		err = da->make_dir_recursive(index_path);
		ERR_FAIL_COND_V_MSG(err != OK, false, "Error: Can't create index directory: " + index_path + ": " + itos(err));
		print_line("Loading classes from: " + index_path);
		err = docsrc.load_classes(index_path);
		ERR_FAIL_COND_V_MSG(err != OK, false, "Error loading classes from: " + index_path + ": " + itos(err));
		checked_paths.insert(index_path);
		print_line("Merging docs...");