GH6848 silently changed series.sort from stable to unstable sort
#6848 is no where mentioned in the release notes. There's a single item on `.order`

changing to quicksort, but it doesn't explicitly warn that the sort was previously stable
and now isn't. There's also no warning about `series.sort` changing as well.
That API has been around forever and afaict nothing was gained by breaking it.  

This was a fairly reckless change in my opinion, I do wish the current maintainers
cared more about backwards-compatibility then they seem to in the latest releases.
