BUG/API: prohibit dtype-changing IntervalArray.__setitem__
- [ ] <s>closes #27147</s> Not quite
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Still needs a dedicated test, but this is also a non-trivial API change, so I want to get the ball rolling on discussion.  cc @jschendel 