Add Dart changelog entry for v25.9.23 (#8785)

Documents changes included in the Dart package v25.9.23 release,
which was published to pub.dev.

Author: vaind

dart/CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 25.9.23
+
+- use enhanced enums (#8313)
+- fix incorrect write in Float64 write method (#8290)
+- code format improvements (#8707)
+
 ## 23.5.26
 
 - omit type annotationes for local variables (#7067, #7069, #7070)