Drop PHP 8.0 support (unsupported by PHP)

What's Changed

• Adjust internal response handling to work with SF6 by @stefandoorn in #262
• Test against PHP 8.3 by @stefandoorn in #264
• Drop PHP 8.0 support (unsupported by PHP) by @stefandoorn in #266

Full Changelog: v2.3.1...v2.4.0