fixes [Vertical stepper shows line after last step](https://github.com/flutter/flutter/issues/144376)

### 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 MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Stepper(
            currentStep: 1,
            steps: const <Step>[
              Step(
                title: Text("Step 1"),
                content: Text("Content 1"),
              ),
              Step(
                title: Text("Step 2"),
                content: Text("Content 2"),
              ),
            ],
          ),
        ),
      ),
    );
  }
}
```

</details>

### Before

![Screenshot 2024-03-25 at 18 36 32](https://github.com/flutter/flutter/assets/48603081/af859a88-7ec8-4432-8eec-f8eb72706b57)

### After
![Screenshot 2024-03-25 at 18 36 24](https://github.com/flutter/flutter/assets/48603081/994325b2-4bd4-44ef-9473-245d3492faf7)