annotation: Prepare to release v4.8.1 (#1310)

Author: kevmoo

json_annotation/CHANGELOG.md

@@ -1,6 +1,7 @@
-## 4.8.1-dev
+## 4.8.1
 
 - Require Dart 2.19
+- Add topics.
 
 ## 4.8.0
 

json_annotation/pubspec.yaml

@@ -1,5 +1,5 @@
 name: json_annotation
-version: 4.8.1-dev
+version: 4.8.1
 description: >-
   Classes and helper functions that support JSON code generation via the
   `json_serializable` package.