BUG: Index._searchsorted_monotonic(..., side='right') returns the left side position for monotonic decreasing indexes
- [X] closes #17271
- [X] closes #16417
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

I didn't add a whatsnew entry for the `Index._searchsorted_monotonic` fix, since it looks like past convention has been to not add whatsnew entries for private methods.  Happy to add a whatsnew entry if I'm mistaken though.  I did add a whatsnew entry for the downstream effect on IntervalIndex.