Deprecate DataFrame.fillna/Series.fillna with method

[j-bennet]

See #10347.

Handles fillna deprecations like this one:

FAILED dask/dataframe/tests/test_dataframe.py::test_fillna - FutureWarning: DataFrame.fillna with 'method' is deprecated and will raise in a future version. Use obj.ffill() or obj.bfill() instead.

• Tests added / passed
• Passes pre-commit run --all-files

The remaining failures are not the same issue.

[phofl]

small comment, otherwise looks generally good to me.

[hendrikmakait]

Overall, the code looks good. I have one open question/suggestion around test readability and one potential new issue we should file.

[hendrikmakait]

Potential follow-up: It looks like we're not validating method anywhere and simply default to bfill even in the case of typos. If I'm correct, we should probably handle bfill in an elif and raise on a "default" else.

[j-bennet]

This preserves the previous behavior:

        if method in ("pad", "ffill"):
            ...
        else:
            method = "bfill"

[hendrikmakait]

This comment is out of scope for this PR, it was more of a general remark about the existing behavior that we may want to fix at some point.

[hendrikmakait]

Thanks, @j-bennet!