Prepare release 6.9 (#1458)

Author: kevmoo

json_serializable/CHANGELOG.md

@@ -1,6 +1,7 @@
-## 6.9.0-wip
+## 6.9.0
 
 - Use conditional map syntax to clean up `null` handling in `toJson` functions.
+- Fix core Dart type links in docs.
 - Require Dart 3.5
 
 ## 6.8.0

json_serializable/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_serializable
-version: 6.9.0-wip
+version: 6.9.0
 description: >-
   Automatically generate code for converting to and from JSON by annotating
   Dart classes.