📝 Add a change note for PR #690

It's added to the `packaging` category as the downstreams are the
ones that would care about this happening most.

docs/changelog-fragments.d/690.packaging.rst

@@ -0,0 +1,9 @@
+The packaging metadata has been migrated to the
+:file:`pyproject.toml`-based :pep:`621` declaration
+-- by :user:`jaraco` and :user:`webknjaz`.
+
+As a part of this update, the minimum version of the
+``setuptools`` build backend was bumped to 61.2.
+Moreover, any compatibility shims that existed in
+:file:`setup.cfg` and :file:`setup.py` have been
+removed for good.