Marked Xamarin as end of support

[viso27]

Fixes #3694

Marks Xamarin-specific techniques as deprecated.
Adds EoS + .NET MAUI note to MASTG-TECH-0125.
Removes Xamarin from minor framework listings.
Updates references to legacy Xamarin where applicable.
Changes follow the guidance from sushi2k and Diolor.

Xamarin is EoS; .NET MAUI is referenced as replacement where relevant.

Fixed the markdown linter error

[github-actions]

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[github-actions]

Automated moderation result.

Verdict, ai-slop.
Confidence, 85.

Reasons.
PR makes sweeping changes across multiple files
Description lacks specific justification for broad edits
No AI disclosure provided
High number of file modifications with minimal context
Action taken based on external guidance without details

Evidence.

• "Marks Xamarin-specific techniques as deprecated.", The description provides minimal context.
• "Changes follow the guidance from sushi2k and Diolor.", External guidance noted but lacks justification for changes.
• "Fixes docs: Mark Xamarin as end of support #3694", Generic reference to an issue number without elaboration.

Suggested action, Request justification for the scope of changes and review evidence..