Drop PHP 8.1 support

What's Changed

• Drop PHP 8.1 support by @jakzal in #517
• Update the required minimum Symfony version by @jakzal in #516
• Update PHPUnit (10.4 -> 11.5.9) by @jakzal in #518
• Allow PHPUnit 12 by @jakzal in #519
• Define property types by @jakzal in #520

Full Changelog: v1.91.0...v1.92.0