Update abandoned Composer packages

[philpem]

Running composer update results in several warnings about abandoned packages:

Package silex/silex is abandoned, you should avoid using it. Use symfony/flex instead.
Package twig/extensions is abandoned, you should avoid using it. No replacement was suggested.
Package symfony/inflector is abandoned, you should avoid using it. Use use `EnglishInflector` from the String component instead instead.
Package swiftmailer/swiftmailer is abandoned, you should avoid using it. Use symfony/mailer instead.
Package facebook/webdriver is abandoned, you should avoid using it. Use php-webdriver/webdriver instead.

These need to be resolved.

[philpem]

Silex: update to Symfony Flex, see https://symfony.com/doc/current/setup/flex.html

Symfony Inflector should require minimal changes, see https://github.com/symfony/symfony/blob/6.1/src/Symfony/Component/String/Inflector/EnglishInflector.php

Swiftmailer -- now Symfony Mailer: https://packagist.org/packages/swiftmailer/swiftmailer

Webdriver -- see https://packagist.org/packages/facebook/webdriver