### 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
* - setup(execpath, argc, argv, p_second_phase) is the main entry point for all platforms,
 *   responsible for the initialization of all low level singletons and core types, and parsing
 *   command line arguments to configure things accordingly.
 *   If p_second_phase is true, it will chain into setup2() (default behavior). This is
 *   disabled on some platforms (Android, iOS, UWP) which trigger the second step in their
 *   own time.
 *
 * - setup2(p_main_tid_override) registers high level servers and singletons, displays the
 *   boot splash, then registers higher level types (scene, editor, etc.).
 *
 * - start() is the last step and that's where command line tools can run, or the main loop
 *   can be created eventually and the project settings put into action. That's also where
 *   the editor node is created, if relevant.
 *   start() does it own argument parsing for a subset of the command line arguments described
 *   in help, it's a bit messy and should be globalized with the setup() parsing somehow.
 */
Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_phase) {
	OS::get_singleton()->initialize();
	engine = memnew(Engine);
	MAIN_PRINT("Main: Initialize CORE");
	engine->startup_begin();
	engine->startup_benchmark_begin_measure("core");
	register_core_types();
	register_core_driver_types();
	MAIN_PRINT("Main: Initialize Globals");
	input_map = memnew(InputMap);
	time_singleton = memnew(Time);
	globals = memnew(ProjectSettings);
	register_core_settings(); 
	translation_server = memnew(TranslationServer);