add whatsnew entry

docs/src/whatsnew/latest.rst

@@ -89,6 +89,10 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ removed a now redundant copying workaround from Resolve testing.
    (:pull:`5267`)
 
+#. `@bjlittle`_ and `@trexfeathers`_ (reviewer) migrated ``setup.cfg`` to
+   ``pyproject.toml``, as motivated by `PEP-0621`_. (:pull:`5262`)
+
+
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,
     core dev names are automatically included by the common_links.inc:
@@ -102,3 +106,4 @@ This document explains the changes made to Iris for this release
 .. _sphinx-panels: https://github.com/executablebooks/sphinx-panels
 .. _sphinx-design: https://github.com/executablebooks/sphinx-design
 .. _check-manifest: https://github.com/mgedmin/check-manifest
+.. _PEP-0621: https://peps.python.org/pep-0621/