DOC: move convert_dtypes whatsnew to 1.0
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
   - moved whatsnew entry from 1.1 to 1.0

Per comment here from @jreback: https://github.com/pandas-dev/pandas/pull/30929#issuecomment-577989924
