Allow Symfony 8 & test PHP 8.5

What's Changed

• Maintenance: GitHub actions, PHP CS Fixer & PHPStan by @jmsche in #20
• Test against PHP 8.4 & 8.5 by @jmsche in #21
• PHPUnit 11 by @jmsche in #22
• Allow Symfony 8 by @jmsche in #23

Full Changelog: v2.0.0...v2.1.0