Update ChangeLog (#508)

Author: kleisauke

CHANGELOG.rst

@@ -1,4 +1,10 @@
-# master 
+## Version 3.0.0 (release date TBD)
+
+- drop support for Python 2.7, require Python >= 3.7 [kleisauke]
+- migrate to `pyproject.toml` [kleisauke]
+- fix a small memleak in `write_to_buffer()` [kleisauke]
+
+## Version 2.2.3 (released 28 April 2024)
 
 - ensure compatibility with a single shared libvips library [kleisauke]
 - add flags_dict(), enum_dict() for better flags introspection