Enable upgrade to PHP 8.0

[webdevel]

This breaks backward compatibility which seems is not avoidable. Enable upgrade to PHP 8.0, Symfony 5.2, PHPUnit 8.5 and 9.5.

PHP Maintained Versions https://www.php.net/supported-versions.php

• 8.0
• 7.4
• 7.3 Symfony Maintained Versions https://symfony.com/releases
• 4.4
• 5.2
• 5.3 dev PHPUnit Supported Versions https://phpunit.de/supported-versions.html
• 8.5
• 9.5

I have an open PR here for driver-testsuite https://github.com/minkphp/driver-testsuite/pull/46.

Please consider proposal of merging into master for a major release breaking backward compatibility with deprecated versions no longer supported or maintained upstream. Thank you for your consideration.

[stof]

PHP 8 support in the CI is done in #158. The rest of this PR is dropping support for Symfony 4, which is a no-go.