Deprecate \Drupal\path_alias\AliasManager::cacheClear() in favour of cachetags

While we're here - what about setCacheKey() and writeCache()? These public methods are:

1. Implemented in AliasManager
2. Not defined in AliasManagerInterface
3. Used in PathAliasSubscriber via a member defined as an instance of AliasManagerInterface

In other words, if you were to implement your own alias manager using the interface without referencing the internals of the core implementation - it would blow up.

No, I'm not actually trying to implement my own alias manager - I'm decorating AliasManager, and phpstan is (rightfully) unhappy with the situation:

  32     Method Drupal\me_performance\MePerformanceAliasManager::setCacheKey() has parameter $key with no type specified.
  33     Call to an undefined method Drupal\path_alias\AliasManagerInterface::setCacheKey().
  40     Call to an undefined method Drupal\path_alias\AliasManagerInterface::writeCache().
  53     Method Drupal\me_performance\MePerformanceAliasManager::cacheClear() has parameter $source with no type specified.

These methods were originally defined in `CacheDecoratorInterface` - when that was deprecated → , the methods probably should have been moved to `AliasManagerInterface`.

📌 AliasManager should use the current route match for outbound alias pre-loading cache keys Active affects this issue - if we do that, some of these methods might even go altogether.