### 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
print_line(String(VERSION_NAME) + " v" + get_full_version_string() + " - " + String(VERSION_WEBSITE));
	engine->startup_benchmark_begin_measure("servers");
	tsman = memnew(TextServerManager);
	if (tsman) {
		Ref<TextServerDummy> ts;
		ts.instantiate();
		tsman->add_interface(ts);
	}
	physics_server_3d_manager = memnew(PhysicsServer3DManager);
	physics_server_2d_manager = memnew(PhysicsServer2DManager);
	register_server_types();
	initialize_modules(MODULE_INITIALIZATION_LEVEL_SERVERS);
	GDExtensionManager::get_singleton()->initialize_extensions(GDExtension::INITIALIZATION_LEVEL_SERVERS);
	if (p_main_tid_override) {
		Thread::main_thread_id = p_main_tid_override;
	}
#ifdef TOOLS_ENABLED
	if (editor || project_manager || cmdline_tool) {
		EditorPaths::create();