Implement time-based cache invalidation for dataset description cache

[emphor11]

### Problem
The function _get_dataset_description caches the dataset description (description.xml) to disk, but the cache is never invalidated.

Dataset descriptions contain metadata such as dataset status (active, deactivated, etc.), which may change over time on the OpenML server. Because the cached description file is reused indefinitely, users may receive stale metadata if the local cache is old.

There is also a TODO comment in the code suggesting that this cache should invalidate itself after some time.

Expected Behavior

Cached dataset descriptions should be refreshed periodically to ensure that metadata reflects the current state on the server.

Current Behavior

Once description.xml is downloaded and cached, it is reused indefinitely unless the cache directory is manually deleted or force_refresh_cache=True is used.

[emphor11]

/assign