6.0.0 - PHP 8.1–8.5 compatibility

WARNING: PHP 8.1 is now the minimum. Now requires ParadoxLabs_TokenBase 5.0.

• Added support up to PHP 8.5; PHP 8.1+ is now required.
• Added unit test coverage.
• Fixed billing address handling on API card save to build a customer AddressInterface (type-safety fix).
• Refactored for PHP 8.1+: constructor property promotion, readonly properties, strict types, import cleanup, and $escaper usage in templates.

Full Changelog: 5.2.0...6.0.0