Added deprecation warning for .label_encoding()

[mayankanand007]

This PR addresses issue #8608 by adding a deprecation warning before we remove the functionality entirely.

[codecov]

Codecov Report

Merging #9289 (12a4dc1) into branch-21.12 (ab4bfaa) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@               Coverage Diff                @@
##           branch-21.12    #9289      +/-   ##
================================================
+ Coverage         10.79%   10.83%   +0.04%     
================================================
  Files               116      116              
  Lines             18869    19258     +389     
================================================
+ Hits               2036     2087      +51     
- Misses            16833    17171     +338     

Impacted Files	Coverage Δ
python/cudf/cudf/core/series.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/_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)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/feather.py	0.00% <0.00%> (ø)
... and 47 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 e2098e5...12a4dc1. Read the comment docs.

[shwina]

@gpucibot merge