### 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
OS::get_singleton()->print("Missing --xr-mode argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--startup-benchmark") {
			use_startup_benchmark = true;
		} else if (I->get() == "--startup-benchmark-file") {
			if (I->next()) {
				use_startup_benchmark = true;
				startup_benchmark_file = I->next()->get();
				N = I->next()->next();
			} else {
				OS::get_singleton()->print("Missing <path> argument for --startup-benchmark-file <path>.\n");
				goto error;
			}
		} else if (I->get() == "--" || I->get() == "++") {
			adding_user_args = true;
		} else {
			main_args.push_back(I->get());
		}
		I = N;
	}
#ifdef TOOLS_ENABLED
	if (editor && project_manager) {
		OS::get_singleton()->print(
				"Error: Command line arguments implied opening both editor and project manager, which is not possible. Aborting.\n");
		goto error;
	}
#endif