docs: Mark Xamarin as end of support

[viso27]

Xamarin has reached End of Support.

This PR updates documentation to reflect Xamarin EoS and removes it from active framework mentions.

Closes #3689

[github-actions]

Thank you for your contribution! However, you are not assigned to any of the linked issues:

• Mark Xamarin as end of support #3689: Mark Xamarin as end of support

To contribute to this project, please:

1. Request to be assigned to the issue you want to work on
2. Wait for a maintainer to assign you
3. Reopen this PR once you are assigned

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[github-actions]

Automated moderation result.

Verdict, ai-slop.
Confidence, 75.

Reasons.
Minimal description of changes
No specific justification for breadth of edits
Unreviewed bulk edits across multiple files

Evidence.

• "This PR updates documentation to reflect Xamarin EoS", The description does not clarify the scope of the changes.
• "removes it from active framework mentions", Suggests a sweeping change without detailed justification.
• Total of 12 files modified with significant changes but unclear rationale.

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