Upgrading .NET MAUI from .NET 7 to .NET 8

[davidbritch]

Add a doc on upgrading from .NET MAUI 7 to .NET MAUI 8. This involves adding explicit package references to the .NET MAUI NuGet packages.

There are three main changes from .NET 7 to 8:

1. NuGet packages.
2. Removal of $(ApplicationIdGuid).
3. Enabling nullables.

https://github.com/dotnet/maui/wiki/Upgrading-.NET-MAUI-from-.NET-7-to-.NET-8

While RC templates set MacCatalyst entitlements, the final GA bits don't.

---

Associated WorkItem - 177240