### 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
OS::get_singleton()->print("  --no-docbase                      Disallow dumping the base types (used with --doctool).\n");
	OS::get_singleton()->print("  --build-solutions                 Build the scripting solutions (e.g. for C# projects). Implies --editor and requires a valid project to edit.\n");
	OS::get_singleton()->print("  --dump-gdextension-interface      Generate GDExtension header file 'gdextension_interface.h' in the current folder. This file is the base file required to implement a GDExtension.\n");
	OS::get_singleton()->print("  --dump-extension-api              Generate JSON dump of the Godot API for GDExtension bindings named 'extension_api.json' in the current folder.\n");
	OS::get_singleton()->print("  --startup-benchmark               Benchmark the startup time and print it to console.\n");
	OS::get_singleton()->print("  --startup-benchmark-file <path>   Benchmark the startup time and save it to a given file in JSON format.\n");
#ifdef TESTS_ENABLED
	OS::get_singleton()->print("  --test [--help]                   Run unit tests. Use --test --help for more information.\n");
#endif
#endif
	OS::get_singleton()->print("\n");
}
#ifdef TESTS_ENABLED
Error Main::test_setup() {
	OS::get_singleton()->initialize();
	engine = memnew(Engine);
	register_core_types();
	register_core_driver_types();
	packed_data = memnew(PackedData);
	globals = memnew(ProjectSettings);
	register_core_settings(); 
	translation_server = memnew(TranslationServer);
	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);