Add PHP 8 support

[nikolaposa]

Since PHP-CS-Fixer blocks adding support for PHP 8, decent alternative is to be installed via Phive.

• [x] add Phive configuration
• [x] remove php-cs-fixer from dev dependencies
• [x] update GitHub Actions build.xml

[nikolaposa]

Closing in favor of a more simpler change for allowing PHP 8 installations: https://github.com/nikolaposa/version/pull/35, after PHP CS-Fixer added PHP 8 support.