### 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
static bool init_use_custom_screen = false;
static Vector2 init_custom_pos;
static bool use_debug_profiler = false;
#ifdef DEBUG_ENABLED
static bool debug_collisions = false;
static bool debug_paths = false;
static bool debug_navigation = false;
#endif
static int frame_delay = 0;
static bool disable_render_loop = false;
static int fixed_fps = -1;
static MovieWriter *movie_writer = nullptr;
static bool disable_vsync = false;
static bool print_fps = false;
#ifdef TOOLS_ENABLED
static bool dump_gdextension_interface = false;
static bool dump_extension_api = false;
#endif
bool profile_gpu = false;
static const String NULL_DISPLAY_DRIVER("headless");
static const String NULL_AUDIO_DRIVER("Dummy");
bool Main::is_cmdline_tool() {
	return cmdline_tool;
}
#ifdef TOOLS_ENABLED
const Vector<String> &Main::get_forwardable_cli_arguments(Main::CLIScope p_scope) {
	return forwardable_cli_arguments[p_scope];
}
#endif
static String unescape_cmdline(const String &p_str) {