DI config: renamed the `old_config` prefix to `ini`

Initially we considered dropping the `ini` config (which lead to the `old_config` name), but the current stance is we keep `config.ini.php` because it's a user friendly config.

I switched to `ini` because it's shorter and maybe more explicit. And it doesn't imply anymore that INI config is deprecated.

Reminder: this feature allows to get `Config` entries from the container (e.g. `$container->get('ini.log.string_message_format')`). The `Config` reads from INI files.