Revise shuffle deprecation to align with dask/dask

[rjzamora]

Description

It seems that #14708 was a bit too aggressive in updating the shuffle kwarg to shuffle_method, and gpu-CI is currently failing in dask/dask.

This PR aligns dask-cudf with dask.dataframe by warning the user when the shuffle argument is used (rather than raising an error). I definitely don't think it makes sense for RAPIDS to be more strict than dask/dask about this argument.

This also fixes a bug in groupby.aggregate() (where we were still using shuffle=)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[rjzamora]

cc @pentschev (Thanks for tackling the original fix! Let me know if you have any thoughts on this small update)

[rjzamora]

CI failures seem unrelated

[jameslamb]

I think this would probably be solved by just re-running the failed jobs:

RuntimeError: Download error (28) Timeout was reached [https://conda.anaconda.org/rapidsai/noarch/repodata.json.zst]
    Operation too slow. Less than 30 bytes/sec transferred the last 60 seconds

Looks like temporary outage in services from anaconda.org.

raft saw something similar around the same time: https://github.com/rapidsai/raft/actions/runs/7546779797/job/20549033091?pr=2096

[jakirkham]

Approving to unblock so we can fix CI

[bdice]

/merge

[jakirkham]

Thanks all! 🙏

Please feel free to submit follow up work if needed. Just want to unblock other PRs

[pentschev]

Sorry I missed this yesterday. Changes look fine to me nevertheless, thanks @rjzamora !