Minor version of Yii Definitions package was released.

• Change PHP constraint in composer.json to ~8.1.0 || ~8.2.0 || ~8.3.0 || ~8.4.0
• Bump minimal required PHP version to 8.1
• Improve definition validation for readonly properties and properties with asymmetric visibility
• Minor performance optimization: use FQN for PHP functions, remove unnecessary conditions
• Mark readonly properties
• Explicitly mark nullable parameters