Xamarin - update references - EntityFramework.Docs

[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 https://github.com/dotnet/EntityFramework.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.

Currently there are around 9 files referencing "Xamarin" and some app samples.

This Get Started with EF Core & Xamarin tutorial requires a major update along with its app samples, likely to be archived, tracking it separately:
dotnet/EntityFramework.Docs#4935
Xamarin Samples (EFGetStarted.Android, EFGetStarted.IOS, EFGetStarted) are located here:
https://github.com/dotnet/EntityFramework.Docs/tree/main/samples/core/Xamarin

The plan topics for EF Core 5 mention Xamarin. To keep the integrity of what was in these announcements at the time, I left mention of Xamarin in the, but added a !Important note indicating the change from Xamarin to .NET MAUI and a link on how to upgrade from Xamarin to .NET MAUI.
Plan for Entity Framework Core 5.0

See also:
#34324, #34325 for AspNetCore.Docs repo efforts
#34326 for the AspNetDocs repo effort.

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Associated WorkItem - 365049