C# Template Fix: File scoped namespaces

[michael-hawker]

Describe the bug

C# 10 introduced file scoped namespaces: https://learn.microsoft.com/dotnet/csharp/language-reference/keywords/namespace

Since the Windows App SDK now targets .NET 6 (which has C# 10 as a default), the templates should be updated to use this new pattern and save a level of indentation in the file.

Follow-on request, related to template clean-up #3324; FYI @bpulliam

Steps to reproduce the bug

Create a new C# project

Expected behavior

namespace MyNamespace;

/// <summary>
/// An empty window that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();
    }

Screenshots

No response

NuGet package version

Windows App SDK 1.2.2: 1.2.221209.1

Packaging type

No response

Windows version

No response

IDE

Visual Studio 2022

Additional context

No response