### 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
#include "core/io/file_access_pack.h"
#include "core/io/file_access_zip.h"
#include "core/io/image_loader.h"
#include "core/io/ip.h"
#include "core/io/resource_loader.h"
#include "core/object/message_queue.h"
#include "core/os/os.h"
#include "core/os/time.h"
#include "core/register_core_types.h"
#include "core/string/translation.h"
#include "core/version.h"
#include "drivers/register_driver_types.h"
#include "main/app_icon.gen.h"
#include "main/main_timer_sync.h"
#include "main/performance.h"
#include "main/splash.gen.h"
#include "modules/register_module_types.h"
#include "platform/register_platform_apis.h"
#include "scene/main/scene_tree.h"
#include "scene/main/window.h"
#include "scene/register_scene_types.h"
#include "scene/resources/packed_scene.h"
#include "scene/theme/theme_db.h"
#include "servers/audio_server.h"
#include "servers/camera_server.h"
#include "servers/display_server.h"
#include "servers/movie_writer/movie_writer.h"
#include "servers/movie_writer/movie_writer_mjpeg.h"
#include "servers/navigation_server_2d.h"
#include "servers/navigation_server_3d.h"