Resolve hash_columns FutureWarning in dask_cudf

[pentschev]

Resolve the following FutureWarning raised in dask_cudf when using safe_hash:

FutureWarning: The `hash_columns` method will be removed in a future cuDF release. Replace `df.hash_columns(columns, method)` with `df[columns].hash_values(method)`.

This blocks Dask-CUDA CI, as we're following a strict policy towards deprecation warnings.

[codecov]

Codecov Report

Merging #9481 (08ea647) into branch-21.12 (ab4bfaa) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9481      +/-   ##
================================================
- Coverage         10.79%   10.64%   -0.15%     
================================================
  Files               116      117       +1     
  Lines             18869    19760     +891     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17656     +823     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
... and 61 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb98505...08ea647. Read the comment docs.

[galipremsagar]

@gpucibot merge

[pentschev]

Thanks @galipremsagar for review/merge!