Deprecate DataFrame.label_encoding, use private _label_encoding method internally.

[bdice]

This PR addresses part of #9474 by deprecating DataFrame.label_encoding and adding a private method that can be used internally without raising a FutureWarning until the public API is removed and internals are refactored. See #9474 for more information.

[codecov]

Codecov Report

Merging #9535 (672a428) into branch-21.12 (ab4bfaa) will decrease coverage by 0.13%.
The diff coverage is n/a.

❗ Current head 672a428 differs from pull request most recent head d47afaf. Consider uploading reports for the commit d47afaf to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9535      +/-   ##
================================================
- Coverage         10.79%   10.65%   -0.14%     
================================================
  Files               116      117       +1     
  Lines             18869    19332     +463     
================================================
+ Hits               2036     2059      +23     
- Misses            16833    17273     +440     

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/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/reshape.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/dtypes.py	0.00% <0.00%> (ø)
... and 42 more

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[galipremsagar]

@gpucibot merge