### 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
doc.merge_from(docsrc);
		for (const String &E : checked_paths) {
			print_line("Erasing old docs at: " + E);
			err = DocTools::erase_classes(E);
			ERR_FAIL_COND_V_MSG(err != OK, false, "Error erasing old docs at: " + E + ": " + itos(err));
		}
		print_line("Generating new docs...");
		err = doc.save_classes(index_path, doc_data_classes);
		ERR_FAIL_COND_V_MSG(err != OK, false, "Error saving new docs:" + itos(err));
		OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		return false;
	}
	if (dump_gdextension_interface) {
		GDExtensionInterfaceDump::generate_gdextension_interface_file("gdextension_interface.h");
	}
	if (dump_extension_api) {
		GDExtensionAPIDump::generate_extension_json_file("extension_api.json");
	}
	if (dump_gdextension_interface || dump_extension_api) {
		OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
		return false;
	}
	if (converting_project) {
		int exit_code = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).convert();
		OS::get_singleton()->set_exit_code(exit_code);
		return false;
	}
	if (validating_converting_project) {
		int exit_code = ProjectConverter3To4(converter_max_kb_file, converter_max_line_length).validate_conversion();
		OS::get_singleton()->set_exit_code(exit_code);