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