### 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
boot_logo.instantiate();
			boot_logo->initialize_data(1, 1, false, Image::FORMAT_RGBA8);
			boot_logo->set_pixel(0, 0, Color(0, 0, 0, 0));
		}
		Color boot_bg_color = GLOBAL_GET("application/boot_splash/bg_color");
#if defined(TOOLS_ENABLED) && !defined(NO_EDITOR_SPLASH)
		boot_bg_color =
				GLOBAL_DEF_BASIC("application/boot_splash/bg_color",
						(editor || project_manager) ? boot_splash_editor_bg_color : boot_splash_bg_color);
#endif
		if (boot_logo.is_valid()) {
			RenderingServer::get_singleton()->set_boot_image(boot_logo, boot_bg_color, boot_logo_scale,
					boot_logo_filter);
		} else {
#ifndef NO_DEFAULT_BOOT_LOGO
			MAIN_PRINT("Main: Create bootsplash");
#if defined(TOOLS_ENABLED) && !defined(NO_EDITOR_SPLASH)
			Ref<Image> splash = (editor || project_manager) ? memnew(Image(boot_splash_editor_png)) : memnew(Image(boot_splash_png));
#else
			Ref<Image> splash = memnew(Image(boot_splash_png));
#endif
			MAIN_PRINT("Main: ClearColor");
			RenderingServer::get_singleton()->set_default_clear_color(boot_bg_color);
			MAIN_PRINT("Main: Image");
			RenderingServer::get_singleton()->set_boot_image(splash, boot_bg_color, false);
#endif
		}
#if defined(TOOLS_ENABLED) && defined(MACOS_ENABLED)
		if (OS::get_singleton()->get_bundle_icon_path().is_empty()) {
			Ref<Image> icon = memnew(Image(app_icon_png));