DOC: move whatsnew file for pandas-dev#21116 (index droplevel)

Author: jorisvandenbossche

doc/source/whatsnew/v0.23.1.txt

@@ -15,8 +15,6 @@ and bug fixes. We recommend that all users upgrade to this version.
 New features
 ~~~~~~~~~~~~
 
-- :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
-
 
 .. _whatsnew_0231.deprecations:
 

doc/source/whatsnew/v0.24.0.txt

@@ -15,7 +15,8 @@ Other Enhancements
 - :func:`to_datetime` now supports the ``%Z`` and ``%z`` directive when passed into ``format`` (:issue:`13486`)
 - :func:`Series.mode` and :func:`DataFrame.mode` now support the ``dropna`` parameter which can be used to specify whether NaN/NaT values should be considered (:issue:`17534`)
 - :func:`to_csv` now supports ``compression`` keyword when a file handle is passed. (:issue:`21227`)
--
+- :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
+
 
 .. _whatsnew_0240.api_breaking: