Update WikiANN data URL
We have to update the URL in the XTREME benchmark as well:

https://github.com/huggingface/datasets/blob/0dfc639cec450ed8762a997789a2ed63e63cdcf2/datasets/xtreme/xtreme.py#L411-L411


WikiANN data source URL is no longer accessible: 404 error from Dropbox.

We have decided to host it at Hugging Face. This PR updates the data source URL, the metadata JSON file and the dataset card.

Close #2691.