Restore PHP 8.1 support

[JanWennrich]

We'd like to use your library in our project, but we have to support PHP 8.1 and thus can't use your library.
This PR restores PHP 8.1 compatibility.

PHP 8.1 receives security support until 31 Dec 2025: https://www.php.net/supported-versions.php

According to packagist, around 32 % of sites, that use a supported PHP version, use PHP 8.1: https://packagist.org/php-statistics

The codebase of this project is compatible with PHP 8.1, as reported by php-parallel-lint:

❯ vendor/bin/parallel-lint src/ tests/
PHP 8.1.29 | 10 parallel jobs
.....                                                        5/5 (100%)

Checked 5 files in 0.1 seconds
No syntax error found

Just PHPUnit 11 is incompatible with PHP 8.1.
Therefore I downgraded PHPUnit to version 10.
The test suite works fine with PHPUnit 10.
Furthermote PHPUnit 10 is still supported: https://phpunit.de/supported-versions.html

[digilist]

Thank you! Yes, I think we can adjust this.

[digilist]

Thank you! I've tagged and released v0.4.2 with these changes.