Bump to PHP 8.1 and upgrade CI to PHPUnit 10

Not PHPUnit 11 because we still need to support for PHP 8.1

e.g. at the moment we have stuff like:

> 1) Symfony\Flex\Tests\Configurator\CopyFromPackageConfiguratorTest::testConfigureAndOverwriteFiles
The at() matcher has been deprecated. It will be removed in PHPUnit 10. Please refactor your test to not rely on the order in which methods are invoked.

Help wanted 🙏 