Update AppHost project documentation for Aspire 13.0 SDK format

[Copilot]

Fixes #5316

Updates the AppHost project section in app-host-overview.md to reflect the new simplified SDK format introduced in Aspire 13.0 (as documented in dotnet/aspire#11502).

Changes

New simplified SDK format (primary example)

Starting with Aspire 13.0, the Aspire.AppHost.Sdk can be set as the sole project SDK, which implicitly adds the Aspire.Hosting.AppHost package reference:

<Project Sdk="Aspire.AppHost.Sdk/13.0.0">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net10.0</TargetFramework>
    </PropertyGroup>
</Project>

Backward compatibility note

Added a note section documenting that the alternative approach of explicitly listing the SDK and package reference still works:

<Project Sdk="Microsoft.NET.Sdk">
    <Sdk Name="Aspire.AppHost.Sdk" Version="13.0.0" />
    
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net10.0</TargetFramework>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Aspire.Hosting.AppHost" Version="13.0.0" />
    </ItemGroup>
</Project>

Version updates

• Updated SDK version references from 9.5.1 to 13.0.0
• Updated target framework from net9.0 to net10.0

This makes it clear that existing projects don't need to be changed to use the new format, while new projects created with the updated templates will use the simplified SDK format.

Original prompt

Please address #5316 - taking into account dotnet/aspire#11502, update app-host-overview.md. Please note that there's another WIP draft PR changing the same file: #5318

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/app-host-overview.md	docs/fundamentals/app-host-overview