maint(gui): Upgrade flutter version (#845)

And major versions of some dependencies that were previously held due
inter-package constraints.

For instance grpc 4.0.0 requires minimum Dart version 3.2.0, but
agentapi was holding Dart max version to 3.0.0, thus dependabot couldn't
upgrade grpc.

Nothing required changes in the proto-generated Dart code.

agentapi/dart/pubspec.yaml

@@ -3,8 +3,8 @@ description: Exposes the Windows Agent gRPC API.
 version: 0.0.1
 
 environment:
-  sdk: '>=2.19.0 <3.0.0'
+  sdk: ">=3.0.0 <4.0.0"
 
 dependencies:
-  grpc: ^3.2.1
+  grpc: ^4.0.0
   protobuf: ^3.0.0

gui/packages/ubuntupro/lib/app.dart

@@ -110,15 +110,15 @@ class Pro4WSLApp extends StatelessWidget {
 
 ThemeData? customize(ThemeData? theme) {
   if (theme == null) return null;
-  const padding = MaterialStatePropertyAll<EdgeInsetsGeometry>(
+  const padding = WidgetStatePropertyAll<EdgeInsetsGeometry>(
     EdgeInsets.symmetric(vertical: 20.0, horizontal: 16.0),
   );
-  const shape = MaterialStatePropertyAll<RoundedRectangleBorder>(
+  const shape = WidgetStatePropertyAll<RoundedRectangleBorder>(
     RoundedRectangleBorder(
       borderRadius: BorderRadius.zero,
     ),
   );
-  final textStyle = MaterialStatePropertyAll<TextStyle>(
+  final textStyle = WidgetStatePropertyAll<TextStyle>(
     theme.textTheme.bodySmall!.copyWith(fontWeight: FontWeight.normal),
   );
   final filledButtonTheme = FilledButtonThemeData(

gui/packages/ubuntupro/lib/pages/subscribe_now/subscribe_now_page.dart

@@ -5,8 +5,10 @@ import 'package:p4w_ms_store/p4w_ms_store.dart';
 import 'package:provider/provider.dart';
 import 'package:ubuntu_service/ubuntu_service.dart';
 import 'package:wizard_router/wizard_router.dart';
-import '../widgets/page_widgets.dart';
+
 import '/core/agent_api_client.dart';
+import '/pages/widgets/page_widgets.dart';
+
 import 'subscribe_now_model.dart';
 import 'subscribe_now_widgets.dart';