Backport PR #57576 on branch 2.2.x (DOC: Add release date for 2.2.1) (#57579)

Backport PR #57576: DOC: Add release date for 2.2.1

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

Author: meeseeksmachine

doc/source/whatsnew/v2.2.1.rst

@@ -1,6 +1,6 @@
 .. _whatsnew_221:
 
-What's new in 2.2.1 (February XX, 2024)
+What's new in 2.2.1 (February 22, 2024)
 ---------------------------------------
 
 These are the changes in pandas 2.2.1. See :ref:`release` for a full changelog