AddApplicationParts Example references deprecated package.

The example provided for AddApplicationParts references code that is deprecated for (at least) .NET 7.
Microsoft.AspNetCore.App has been deprecated.

[see nuget package here](https://www.nuget.org/packages/Microsoft.AspNetCore.App/)

how are we expected to add additional parts to an application now without making the dependent assembly itself an ASP.NET Core project?

---
#### Document Details

⚠ *Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.*

* ID: 2784a724-e940-4c14-3b50-b063f78c2ee5
* Version Independent ID: 41175c3a-8cb5-856b-a7e0-931edb6394db
* Content: [Share controllers, views, Razor Pages and more with Application Parts in ASP.NET Core](https://learn.microsoft.com/en-us/aspnet/core/mvc/advanced/app-parts?view=aspnetcore-7.0)
* Content Source: [aspnetcore/mvc/advanced/app-parts.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/mvc/advanced/app-parts.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-mvc**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**