fix: deprecated dynamic property in OpenyRepeatRepository

Problem In PHP 8.2+, assigning values to undeclared class properties triggers a deprecation notice. Currently, the property $memoryCache is being dynamically assigned in OpenyRepeatRepository, which results in the following warning:

Deprecated function: Creation of dynamic property Drupal\openy_repeat\OpenyRepeatRepository::$memoryCache is deprecated in Drupal\openy_repeat\OpenyRepeatRepository->__construct()

Solution Declared the $memoryCache property explicitly in the class to eliminate the deprecation warning and ensure compatibility with PHP 8.2 and above.