Skip non-numeric columns when making a boxplot
This PR updates the `BoxPlot` class to skip non-numeric columns when making a DataFrame boxplot. Also raises a more meaningful error message when trying to plot a datetime-like Series.  

Closes #22799.

- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
