Xamarin - update references - AspNetDocs repo

[wadepickett]

Xamarin should no longer be referred to and we should refer to or direct to Maui instead:
https://learn.microsoft.com/en-us/dotnet/maui/what-is-maui?view=net-maui-9.0

In the AspNetCore.Docs repo:

• Remove all references to "Xamarin" or update to ".NET Multi-platform App UI (.NET MAUI)" for first reference and ".NET MAUI" for following references in the same doc.
• If a topic centers on .NET MAUI and requires a complete rewrite rather than just an updated link or reference update, and/or requires a code sample update, AND we decide not to retire or archive it as the solution, then identify it here and then create a new tracking issue for that work.

Notify David Britch when complete.

There are around 2 files referencing "Xamarin" in this repo.

See also #34324 and #34325 for a similar effort for the AspNetCore.Docs repo