DEPR: Deprecate Series.to_csv signature
- [x] closes #19715
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

<strike>Just a proof of concept for discussion (misses docs, whatsnew, new tests). Based on #21868</strike>

@dahlbaek @gfyoung 