Mark FRAMEWORK_6 as compatible with PHP 8

What's Changed

• Fix classname case by @ralflang in #3
• create a unique cache-file by @cheese1 in #9

New Contributors

• @ralflang made their first contribution in #3
• @cheese1 made their first contribution in #9

Full Changelog: https://github.com/horde/Core/commits/v3.0.0alpha15