REF (string dtype): rename using_pyarrow_string_dtype to using_string_dtype
xref https://github.com/pandas-dev/pandas/issues/54792

This is a plain search-replace of `using_pyarrow_string_dtype` with `using_string_dtype` plus some automated formatting because of the change.

Given the future string dtype will not be strictly backed by pyarrow, this general config helper to check if the future option is enabled can be better renamed (we might need a more specific check if we are using the pyarrow variant in certain cases later, but that can be added when needed), and given there are coming more PRs working on this, better to do this rename first.

Thanks @jorisvandenbossche 