fixes [Screen reader is not announcing the selected date as selected on DatePicker](https://github.com/flutter/flutter/issues/143439)

### Descriptions
- This fixes an issue where `CalendarDatePicker` doesn't announce selected date on desktop.
- Add semantic label to describe the selected date is indeed "Selected".

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  MyHomePageState createState() => MyHomePageState();
}

class MyHomePageState extends State<MyHomePage> {
  void _showDatePicker() async {
    await showDatePicker(
      context: context,
      initialDate: DateTime.now(),
      firstDate: DateTime(1900),
      lastDate: DateTime(2200),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title,
            style: const TextStyle(fontFamily: 'ProductSans')),
      ),
      body: const Center(
        child: Text('Click the button to show date picker.'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _showDatePicker,
        tooltip: 'Show date picker',
        child: const Icon(Icons.edit_calendar),
      ),
    );
  }
}

// import 'package:flutter/material.dart';

// void main() => runApp(const MyApp());

// class MyApp extends StatelessWidget {
//   const MyApp({super.key});

//   @override
//   Widget build(BuildContext context) {
//     return MaterialApp(
//       debugShowCheckedModeBanner: false,
//       home: Scaffold(
//         body: Center(
//           child: CalendarDatePicker(
//             initialDate: DateTime.now(),
//             firstDate: DateTime(2020),
//             lastDate: DateTime(2050),
//             onDateChanged: (date) {
//               print(date);
//             },
//           ),
//         ),
//       ),
//     );
//   }
// }
```

</details>

### Before

https://github.com/flutter/flutter/assets/48603081/c82e1f15-f067-4865-8a5a-1f3c0c8d91da

### After

https://github.com/flutter/flutter/assets/48603081/193d9e26-df9e-4d89-97ce-265c3d564607