ENH: Deprecate positional arguments for DataFrame.fillna and Series.fillna (GH41485)
- [x] xref #41485
- [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



@MarcoGorelli

I was able to find a few hundreds of `DataFrame.fillna` or `Series.fillna` usages, mostly in tests, and didn't see any usages like `srs.fillna(0, None)`  where `method` and other args are passed in directly as positional. Those usages include a few function calls in string format like [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/groupby/test_groupby.py#L2066-L2067) and [here](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/apply/test_frame_transform.py#L42-L52). So please let me know if I missed anywhere.

On a separate note, should positional args for `Block.fillna` also be deprecated? Nice, thanks!

> On a separate note, should positional args for Block.fillna also be deprecated?

The value of `inplace` doesn't change the return type there, so it's not so crucial for overloads, and I think it's only used internally, so I'm pretty sure it doesn't need changing Gotcha!  thanks @jackzyliu 