[Data] Replace deprecated .pieces with updated .fragments

[bveeramani]

Why are these changes needed?

ParquetDataset.pieces has been deprecated since PyArrow 5. This PR replaces it with the updated .fragments attribute.

/tmp/ray/session_2023-09-09_18-08-45_602405_2335/runtime_resources/py_modules_files/_ray_pkg_16a18957f9722be1/ray/data/datasource/parquet_datasource.py:270: FutureWarning: 'ParquetDataset.pieces' attribute is deprecated as of pyarrow 5.0.0 and will be removed in a future version. Use the '.fragments' attribute instead

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[scottjlee]

i think there are some failing doctests, but changes generally LGTM

[amogkam]

thanks for doing this!! these warnings were very annoying

[c21]

Thanks @bveeramani!