update changelogs and pubspecs

packages/stream_chat/pubspec.yaml

@@ -1,7 +1,7 @@
 name: stream_chat
 homepage: https://getstream.io/
 description: The official Dart client for Stream Chat, a service for building chat applications.
-version: 1.5.3
+version: 2.0.0-nullsafety.0
 repository: https://github.com/GetStream/stream-chat-flutter
 issue_tracker: https://github.com/GetStream/stream-chat-flutter/issues
 
@@ -24,8 +24,8 @@ dependencies:
   web_socket_channel: ^2.0.0
 
 dev_dependencies:
-  build_runner: ^1.12.2
-  freezed: ^0.14.1+2
+  build_runner: ^2.0.1
+  freezed: ^0.14.1+3
   json_serializable: ^4.1.0
   mocktail: ^0.1.1
   test: ^1.16.8