### 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 (found_project && EditorPaths::get_singleton()->is_self_contained()) {
			if (ProjectSettings::get_singleton()->get_resource_path() == OS::get_singleton()->get_executable_path().get_base_dir()) {
				ERR_PRINT("You are trying to run a self-contained editor at the same location as a project. This is not allowed, since editor files will mix with project files.");
				OS::get_singleton()->set_exit_code(EXIT_FAILURE);
				return FAILED;
			}
		}
	}
#endif
	
	input = memnew(Input);
	
	{
		String display_driver = DisplayServer::get_create_function_name(display_driver_idx);
		Vector2i *window_position = nullptr;
		Vector2i position = init_custom_pos;
		if (init_use_custom_pos) {
			window_position = &position;
		}
		Color boot_bg_color = GLOBAL_DEF_BASIC("application/boot_splash/bg_color", boot_splash_bg_color);
		DisplayServer::set_early_window_clear_color_override(true, boot_bg_color);
		
		Error err;
		display_server = DisplayServer::create(display_driver_idx, rendering_driver, window_mode, window_vsync_mode, window_flags, window_position, window_size, init_screen, err);
		if (err != OK || display_server == nullptr) {
			
			
			
			for (int i = 0; i < DisplayServer::get_create_function_count() - 1; i++) {
				if (i == display_driver_idx) {