Drop PHP 8.1 support

What's Changed

• Drop PHP 8.1 support by @jakzal in jakzal/toolbox#517

• Update the required minimum Symfony version by @jakzal in jakzal/toolbox#516

• Update PHPUnit (10.4 -> 11.5.9) by @jakzal in jakzal/toolbox#518

• Allow PHPUnit 12 by @jakzal in jakzal/toolbox#519

• Define property types by @jakzal in jakzal/toolbox#520

Full Changelog: jakzal/toolbox@v1.91.0...v1.92.0