ENH: Make Series.update() use objects coercible to Series
- [x] closes #33215 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Allows `Series.update()` to accept objects that can be coerced into a `Series`, like a `dict`. This mirrors the existing behavior of `DataFrame.update()`.