4.0.0

This version enforces PHP 8.1. Please only use ^4.0.0 with this version of PHP. ^3.0.0 will still be supported for the time being and works on PHP 7.4 and 8.0 (without deprecation warnings).

• Removes myclabs/php-enum in favor of native PHP enums.
• Utilizes constructor property promotion where appropriate (e.g. in constructors with only one or maybe two parameters; for clarity reasons).
• Utilizes mixed type where applicable.
• Upgrades OAuth client to the new one Sony is using.