Update datasets to 2.20.0

[albertvillanova]

Update datasets to 2.20.0.

This PR is intended to address the CI errors raised by this update, as a first step before:

• Update datasets to 2.21.0 #3037

Fixes after the update of datasets:

• Pass trust_remote_code=True for script dataset: 227f2c4
• Use JSON-Lines (instead of JSON) dataset in test_statistics_endpoint to avoid pandas bug that downcasts float to int column: a86d040

[albertvillanova]

EDIT:
I have replaced the JSON dataset with a JSON-Lines dataset in test_statistics_endpoint: a86d040

• JSON-Lines files are read with pyarrow instead of pandas

Note I have commented an assertion in test_statistics_endpoint: c437c9f

• when reading a JSON file, pandas downcasts a float column to int if all the numbers can be downcast to integers, e.g. [1.0, 2.0, 3.0] -> [1, 2, 3]
• I don't think this is a big issue