DEPR: Warn about Series.to_csv signature alignment
Warns about aligning `Series.to_csv`'s signature with that of `DataFrame.to_csv`'s.

In anticipation, we have moved `DataFrame.to_csv` to `generic.py` so that we can later delete the
`Series.to_csv` implementation, and allow it to adopt `DataFrame`'s `to_csv` due to inheritance.

Closes #19745.

cc @dahlbaek