BUG: Change default of observed in Series.groupby (#62025)

Author: rhshadrach

pandas/core/series.py

@@ -1972,7 +1972,7 @@ def groupby(
         as_index: bool = True,
         sort: bool = True,
         group_keys: bool = True,
-        observed: bool = False,
+        observed: bool = True,
         dropna: bool = True,
     ) -> SeriesGroupBy:
         from pandas.core.groupby.generic import SeriesGroupBy