### 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
memdelete(theme_db);
	theme_db = nullptr;
}
#ifdef DEBUG_INIT
#define MAIN_PRINT(m_txt) print_line(m_txt)
#else
#define MAIN_PRINT(m_txt)
#endif
void Main::print_help(const char *p_binary) {
	print_line(String(VERSION_NAME) + " v" + get_full_version_string() + " - " + String(VERSION_WEBSITE));
	OS::get_singleton()->print("Free and open source software under the terms of the MIT license.\n");
	OS::get_singleton()->print("(c) 2014-present Godot Engine contributors.\n");
	OS::get_singleton()->print("(c) 2007-2014 Juan Linietsky, Ariel Manzur.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Usage: %s [options] [path to scene or 'project.godot' file]\n", p_binary);
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("General options:\n");
	OS::get_singleton()->print("  -h, --help                        Display this help message.\n");
	OS::get_singleton()->print("  --version                         Display the version string.\n");
	OS::get_singleton()->print("  -v, --verbose                     Use verbose stdout mode.\n");
	OS::get_singleton()->print("  -q, --quiet                       Quiet mode, silences stdout messages. Errors are still displayed.\n");
	OS::get_singleton()->print("\n");
	OS::get_singleton()->print("Run options:\n");
	OS::get_singleton()->print("  --, ++                            Separator for user-provided arguments. Following arguments are not used by the engine, but can be read from `OS.get_cmdline_user_args()`.\n");
#ifdef TOOLS_ENABLED
	OS::get_singleton()->print("  -e, --editor                      Start the editor instead of running the scene.\n");
	OS::get_singleton()->print("  -p, --project-manager             Start the project manager, even if a project is auto-detected.\n");
	OS::get_singleton()->print("  --debug-server <uri>              Start the editor debug server (<protocol>://<host/IP>[:<port>], e.g. tcp://127.0.0.1:6007)\n");
#endif
	OS::get_singleton()->print("  --quit                            Quit after the first iteration.\n");