Version 3.1 of Yii Caching Library is released.

• Added interface SerializerInterface for data serialization, and PhpSerializer implementation.
• Made normalize() method static in CacheKeyNormalizer class.
• Changed PHP constraint in composer.json to 8.1 - 8.4.
• Raised the minimum PHP version to 8.1 and minor refactoring.
• Explicitly marked parameters as nullable.