### 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
if (minimum_time_msec) {
		uint64_t minimum_time = 1000 * minimum_time_msec;
		uint64_t elapsed_time = OS::get_singleton()->get_ticks_usec();
		if (elapsed_time < minimum_time) {
			OS::get_singleton()->delay_usec(minimum_time - elapsed_time);
		}
	}
	if (use_startup_benchmark) {
		Engine::get_singleton()->startup_dump(startup_benchmark_file);
		startup_benchmark_file = String();
	}
	return true;
}
/* Main iteration
 *
 * This is the iteration of the engine's game loop, advancing the state of physics,
 * rendering and audio.
 * It's called directly by the platform's OS::run method, where the loop is created
 * and monitored.
 *
 * The OS implementation can impact its draw step with the Main::force_redraw() method.
 */
uint64_t Main::last_ticks = 0;
uint32_t Main::frames = 0;
uint32_t Main::hide_print_fps_attempts = 3;
uint32_t Main::frame = 0;
bool Main::force_redraw_requested = false;
int Main::iterating = 0;
bool Main::agile_input_event_flushing = false;
bool Main::is_iterating() {