Backport PR pandas-dev#55211 on branch 2.1.x (DOC: Add release date for 2.1.1) (pandas-dev#55218)

Backport PR pandas-dev#55211: DOC: Add release date for 2.1.1

Co-authored-by: Thomas Li <[email protected]>

Author: meeseeksmachine

doc/source/whatsnew/v2.1.1.rst

@@ -1,6 +1,6 @@
 .. _whatsnew_211:
 
-What's new in 2.1.1 (September XX, 2023)
+What's new in 2.1.1 (September 20, 2023)
 ----------------------------------------
 
 These are the changes in pandas 2.1.1. See :ref:`release` for a full changelog