### 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 "servers/physics_server_2d.h"
#include "servers/physics_server_3d.h"
#include "servers/register_server_types.h"
#include "servers/rendering/rendering_server_default.h"
#include "servers/text/text_server_dummy.h"
#include "servers/text_server.h"
#include "servers/xr_server.h"
#ifdef TESTS_ENABLED
#include "tests/test_main.h"
#endif
#ifdef TOOLS_ENABLED
#include "editor/debugger/editor_debugger_node.h"
#include "editor/doc_data_class_path.gen.h"
#include "editor/doc_tools.h"
#include "editor/editor_node.h"
#include "editor/editor_paths.h"
#include "editor/editor_settings.h"
#include "editor/editor_translation.h"
#include "editor/progress_dialog.h"
#include "editor/project_converter_3_to_4.h"
#include "editor/project_manager.h"
#include "editor/register_editor_types.h"
#ifndef NO_EDITOR_SPLASH
#include "main/splash_editor.gen.h"
#endif
#endif
#include "modules/modules_enabled.gen.h" 
static Engine *engine = nullptr;
static ProjectSettings *globals = nullptr;
static Input *input = nullptr;