Symfony version(s) affected: 3.4.21

Description
in php 7.1. new session options - sid_length, sid_bits_per_character were added.
http://php.net/session.configuration

There are closed / merged issues that should handle this changes
#21606
#23128
but I can still reproduce error in version 3.4 LTS with php 7.2.13 on Ubuntu 16.04

How to reproduce

1. clear install with symfony installer, create empty project
   https://symfony.com/doc/3.4/setup.html

2. add framework.session.sid_length or sid_bits_per_character properties in /app/config/config.yml

framework:
	...
    session:
        handler_id: session.handler.native_file
        save_path: '%kernel.project_dir%/var/sessions/%kernel.environment%'
        sid_length: 64

1. execute from console from project root
   php bin/console cache:clear --env=prod
   (or open index page in browser)

got:

In ArrayNode.php line 307:
  Unrecognized option "sid_length" under "framework.session"

Possible Solution
Only file affected in #21606 is NativeSessionStorage.php. I see changes in my local file, but anyway I got this error.

    public function setOptions(array $options)
    {
        ...
        $validOptions = array_flip(array(
            ...
            'sid_length', 'sid_bits_per_character', 'trans_sid_hosts', 'trans_sid_tags',
        ));

Additional context

use_only_cookies is "Unrecognized option" too