checked_yaml: allow null-safe json_annotation (#765)

Prepare to release 1.0.3
Switch back to proper casing for CHANGELOG and README

Author: kevmoo

checked_yaml/changelog.md renamed to checked_yaml/CHANGELOG.md

@@ -1,6 +1,7 @@
-## 1.0.3-dev
+## 1.0.3
 
 - Require at least Dart `2.7.0`.
+- Allow `package:json_annotation` `v4.x`.
 
 ## 1.0.2
 

checked_yaml/readme.md renamed to checked_yaml/README.md

@@ -8,7 +8,7 @@ environment:
   sdk: '>=2.7.0 <3.0.0'
 
 dependencies:
-  json_annotation: '>=2.2.0 <4.0.0'
+  json_annotation: '>=2.2.0 <5.0.0'
   source_span: ^1.0.0
   yaml: ^2.1.13