BUG: Remove artificial precision limit in rolling var & std
- [x] closes #37051, xref #40448, xref #39872
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

thanks @mroeschke 