### 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
} else {
				OS::get_singleton()->print("Missing rendering driver argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "-f" || I->get() == "--fullscreen") { 
			init_fullscreen = true;
		} else if (I->get() == "-m" || I->get() == "--maximized") { 
			init_maximized = true;
			window_mode = DisplayServer::WINDOW_MODE_MAXIMIZED;
		} else if (I->get() == "-w" || I->get() == "--windowed") { 
			init_windowed = true;
		} else if (I->get() == "--gpu-index") {
			if (I->next()) {
				Engine::singleton->gpu_idx = I->next()->get().to_int();
				N = I->next()->next();
			} else {
				OS::get_singleton()->print("Missing GPU index argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--gpu-validation") {
			Engine::singleton->use_validation_layers = true;
#ifdef DEBUG_ENABLED
		} else if (I->get() == "--gpu-abort") {
			Engine::singleton->abort_on_gpu_errors = true;
#endif
		} else if (I->get() == "--tablet-driver") {
			if (I->next()) {
				tablet_driver = I->next()->get();
				N = I->next()->next();
			} else {