Fix regression for pandas < 2.0.0 in JSON loader
A regression was introduced for pandas < 2.0.0 in PR:
- #6914

As described in pandas docs, the `dtype_backend` parameter was first added in pandas 2.0.0: https://pandas.pydata.org/docs/reference/api/pandas.read_json.html

This PR fixes the regression by passing (or not) the `dtype_backend` parameter depending on pandas version.

Maybe, in a future 3.0 `datasets` release, we could just require pandas > 2.0.

Reported by:
- #6977