Whats new updates for v3.9.0 . (#5919)

Author: trexfeathers

docs/src/whatsnew/3.9.rst

@@ -1,7 +1,7 @@
 .. include:: ../common_links.inc
 
-v3.9 (03 Apr 2024) [release candidate]
-**************************************
+v3.9 (22 Apr 2024)
+******************
 
 This document explains the changes made to Iris for this release
 (:doc:`View all changes <index>`.)