Remove support for deprecated `XCache` and `ZendDataCache` extensions.

[terabytesoftw]

Yii2 currently includes support for XCache and ZendDataCache caching extensions, but these extensions are no longer maintained and are incompatible with modern PHP versions.

Current State

• XCache development stopped years ago and doesn't support PHP 7.x+.
• ZendDataCache (part of Zend Server) is also deprecated and not available for current PHP versions.
• Yii2 minimum PHP requirement is 7.4.

Files potentially affected

• yii\caching\XCache.
• yii\caching\ZendDataCache.
• Related tests and documentation.

This cleanup will make the codebase more maintainable and remove confusion about supported caching options.