BUG: Fix `skipna` default value in method signatures
Set `skipna` default values to `True` so that the value matches the
purpose of the variable and its docstring.

- [X] closes #34063
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry
