### 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
return false;
	}
#endif
	if (script.is_empty() && game_path.is_empty() && String(GLOBAL_GET("application/run/main_scene")) != "") {
		game_path = GLOBAL_GET("application/run/main_scene");
	}
#ifdef TOOLS_ENABLED
	if (!editor && !project_manager && !cmdline_tool && script.is_empty() && game_path.is_empty()) {
		/
		/
		/
		/
		OS::get_singleton()->alert("Couldn't detect whether to run the editor, the project manager or a specific project. Aborting.");
		ERR_FAIL_V_MSG(false, "Couldn't detect whether to run the editor, the project manager or a specific project. Aborting.");
	}
#endif
	MainLoop *main_loop = nullptr;
	if (editor) {
		main_loop = memnew(SceneTree);
	}
	String main_loop_type = GLOBAL_DEF("application/run/main_loop_type", "SceneTree");
	if (!script.is_empty()) {
		Ref<Script> script_res = ResourceLoader::load(script);
		ERR_FAIL_COND_V_MSG(script_res.is_null(), false, "Can't load script: " + script);
		if (check_only) {
			if (!script_res->is_valid()) {
				OS::get_singleton()->set_exit_code(EXIT_FAILURE);
			} else {
				OS::get_singleton()->set_exit_code(EXIT_SUCCESS);
			}