Drop support for EOL PHP 8.0

[tcarrio]

This includes upgrading to newer language offerings like native enums

• [ ] Drop support for PHP 8.0
• [ ] Refactor to utilize native enums
• [ ] Refactor to read-only properties
• [ ] Refactor to intersection types
• [ ] Refactor to final class constants
• [ ] Refactor to object array unpacking
• [ ] Ensure null parameters are marked as such

Reference: https://www.php.net/releases/8.1/en.php