### 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
performance = memnew(Performance);
	GDREGISTER_CLASS(Performance);
	engine->add_singleton(Engine::Singleton("Performance", performance));
	
	
	GLOBAL_DEF_RST("application/run/flush_stdout_on_print", false);
	GLOBAL_DEF_RST("application/run/flush_stdout_on_print.debug", true);
	MAIN_PRINT("Main: Parse CMDLine");
	
	List<String> args;
	List<String> main_args;
	List<String> user_args;
	bool adding_user_args = false;
	List<String> platform_args = OS::get_singleton()->get_cmdline_platform_args();
	
	for (int i = 0; i < argc; i++) {
		args.push_back(String::utf8(argv[i]));
	}
	
	for (const String &arg : platform_args) {
		args.push_back(arg);
	}
	List<String>::Element *I = args.front();
	while (I) {
		I->get() = unescape_cmdline(I->get().strip_edges());
		I = I->next();
	}
	String display_driver = "";
	String audio_driver = "";
	String project_path = ".";