Create a final release that points to core classes

Now that VariationCache is part of Drupal core 10.2 we want people to use the core version instead. However, until all modules using VariationCache can update their minimum requirement to Drupal 10.2 or greater, we need to make sure that they can still use the old namespace (\Drupal\variationcache\foo) without issues.

Therefore, we should move the current classes to an Old folder and point to the core classes if available. If the core classes do not exist, it means we are on a lower Drupal core version and we can make the old namespaced classes point to the Old folder. All of this is done in the .module file by using class_alias, which is an officially supported practice used for experimental modules in core.

See: this part of the experimental documentation