BUG: Fix Series.sort_values descending & mergesort
The solution uses the `nargsort` function, which was created to handle
in one place the intricacies of sorting. Not adapting Series.sort_values
to use may have been an oversight.

- [ ] closes #28697
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
