### 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
if (adding_user_args) {
			user_args.push_back(I->get());
		} else if (I->get() == "-h" || I->get() == "--help" || I->get() == "/?") { 
			show_help = true;
			exit_code = ERR_HELP; 
			goto error;
		} else if (I->get() == "--version") {
			print_line(get_full_version_string());
			exit_code = ERR_HELP; 
			goto error;
		} else if (I->get() == "-v" || I->get() == "--verbose") { 
			OS::get_singleton()->_verbose_stdout = true;
		} else if (I->get() == "-q" || I->get() == "--quiet") { 
			quiet_stdout = true;
		} else if (I->get() == "--audio-driver") { 
			if (I->next()) {
				audio_driver = I->next()->get();
				bool found = false;
				for (int i = 0; i < AudioDriverManager::get_driver_count(); i++) {
					if (audio_driver == AudioDriverManager::get_driver(i)->get_name()) {
						found = true;
					}
				}
				if (!found) {
					OS::get_singleton()->print("Unknown audio driver '%s', aborting.\nValid options are ",
							audio_driver.utf8().get_data());
					for (int i = 0; i < AudioDriverManager::get_driver_count(); i++) {
						if (i == AudioDriverManager::get_driver_count() - 1) {
							OS::get_singleton()->print(" and ");
						} else if (i != 0) {