Add whatsnew

Author: bouweandela

docs/src/whatsnew/latest.rst

@@ -148,6 +148,8 @@ This document explains the changes made to Iris for this release
 #. `@trexfeathers`_ temporarily pinned Sphinx to `<8.2`.
    (:pull:`6344`, :issue:`6345`)
 
+#. `@bouweandela`_ fixed a bug in the benchmarking code that caused all benchmarks
+    to be run with lazy data. (:pull:`6339`)
 
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,