Mark Xamarin as end of support

[viso27]

"Fixes Mark Xamarin as end of support #3689 "

1. Marked Xamarin-specific techniques (e.g., MASTG-TECH-0125) as deprecated.
2. Added a clear note explaining that Xamarin is End of Support and that .NET MAUI should be used instead for new development.
3. Updated framework listings to remove Xamarin where it was only mentioned as an active option.
4. Clarified references to Xamarin as “legacy” where applicable.
5. Fixed the markdown linter errors

The edits follow the guidance provided by Diolor,sushi2k and cpholguera.

[github-actions]

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