Update package:stager to 1.3.0 (#9498)

Author: elliette

packages/devtools_app/pubspec.yaml

@@ -73,17 +73,12 @@ dev_dependencies:
   integration_test:
     sdk: flutter
   mockito: ^5.4.1
-  stager: ^1.0.1
+  stager: ^1.3.0
   stream_channel: ^2.1.1
   test: ^1.21.0
   web_benchmarks: ^4.0.0
   webkit_inspection_protocol: ">=0.5.0 <2.0.0"
 
-dependency_overrides:
-  # Necessary while `stager` dep disallows `source_gen` 2.0.0. Remove when
-  # `stager` allows `source_gen` 2.0.0.
-  source_gen: ^2.0.0
-
 flutter:
   uses-material-design: true
   assets: