BUG: fix .str.isdigit to honor unicode superscript for older pyarrow
- [x] closes #61466
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

pandas/tests/strings/test_strings.py::test_isnumeric_unicode > pandas/tests/strings/test_strings.py::test_isnumeric_unicode

Yeah, see https://github.com/pandas-dev/pandas/issues/61466#issuecomment-3121827923 (but I suppose the best option is just to accept that difference and update the test to reflect it. Alternatively we could still only use pyarrow for ascii, and always fall back to python for unicode, if we really want consistent behaviour)