Added deprecation warning for .label_encoding() (#9289)

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

Authors:
  - Mayank Anand (https://github.com/mayankanand007)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)

URL: #9289

python/cudf/cudf/core/series.py

@@ -3637,6 +3637,12 @@ def label_encoding(self, cats, dtype=None, na_sentinel=-1):
         dtype: int8
         """
 
+        warnings.warn(
+            "Series.label_encoding is deprecated and will be removed in the future.\
+                 Consider using cuML's LabelEncoder instead",
+            DeprecationWarning,
+        )
+
         def _return_sentinel_series():
             return Series(
                 cudf.core.column.full(