### 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
const double time_scale = Engine::get_singleton()->get_time_scale();
	MainFrameTime advance = main_timer_sync.advance(physics_step, physics_ticks_per_second);
	double process_step = advance.process_step;
	double scaled_step = process_step * time_scale;
	Engine::get_singleton()->_process_step = process_step;
	Engine::get_singleton()->_physics_interpolation_fraction = advance.interpolation_fraction;
	uint64_t physics_process_ticks = 0;
	uint64_t process_ticks = 0;
	uint64_t navigation_process_ticks = 0;
	frame += ticks_elapsed;
	last_ticks = ticks;
	const int max_physics_steps = Engine::get_singleton()->get_max_physics_steps_per_frame();
	if (fixed_fps == -1 && advance.physics_steps > max_physics_steps) {
		process_step -= (advance.physics_steps - max_physics_steps) * physics_step;
		advance.physics_steps = max_physics_steps;
	}
	bool exit = false;
	/
	XRServer::get_singleton()->_process();
	for (int iters = 0; iters < advance.physics_steps; ++iters) {
		if (Input::get_singleton()->is_using_input_buffering() && agile_input_event_flushing) {
			Input::get_singleton()->flush_buffered_events();
		}
		Engine::get_singleton()->_in_physics = true;
		uint64_t physics_begin = OS::get_singleton()->get_ticks_usec();
		PhysicsServer3D::get_singleton()->sync();
		PhysicsServer3D::get_singleton()->flush_queries();
		PhysicsServer2D::get_singleton()->sync();
		PhysicsServer2D::get_singleton()->flush_queries();
		if (OS::get_singleton()->get_main_loop()->physics_process(physics_step * time_scale)) {