### 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 "main.h"
#include "core/config/project_settings.h"
#include "core/core_globals.h"
#include "core/core_string_names.h"
#include "core/crypto/crypto.h"
#include "core/debugger/engine_debugger.h"
#include "core/extension/extension_api_dump.h"
#include "core/extension/gdextension_interface_dump.gen.h"
#include "core/extension/gdextension_manager.h"
#include "core/input/input.h"
#include "core/input/input_map.h"
#include "core/io/dir_access.h"
#include "core/io/file_access_network.h"