### 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
ERR_FAIL_COND_V(!_start_success, false);
	bool hasicon = false;
	String positional_arg;
	String game_path;
	String script;
	bool check_only = false;
#ifdef TOOLS_ENABLED
	String doc_tool_path;
	bool doc_base = true;
	String _export_preset;
	bool export_debug = false;
	bool export_pack_only = false;
	bool converting_project = false;
	bool validating_converting_project = false;
#endif
	main_timer_sync.init(OS::get_singleton()->get_ticks_usec());
	List<String> args = OS::get_singleton()->get_cmdline_args();
	for (int i = 0; i < args.size(); i++) {
		/
		/
		/
		if (args[i] == "--check-only") {
			check_only = true;
#ifdef TOOLS_ENABLED
		} else if (args[i] == "--no-docbase") {
			doc_base = false;
		} else if (args[i] == "--convert-3to4") {