Mark Xamarin as end of support

[viso27]

This PR closes #3689

Description

This PR:

• Marked Xamarin-specific techniques (e.g., MASTG-TECH-0125) as deprecated.

• Added a clear note explaining that Xamarin is End of Support and that .NET MAUI should be used instead for new development.

• Updated framework listings to remove Xamarin where it was only mentioned as an active option.

• Clarified references to Xamarin as “legacy” where applicable.

• Fixed the markdown linter errors

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

AI Tool Disclosure

Check exactly one option.

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Contributor Checklist

• I have read and understood the contributing guidelines.
• I followed the project style guide.
• I validated the technical correctness of my changes and understand the topic.
• This PR adds clear value and is not spam or low-effort content.

Relevant documentation.

• General Contributing Guidelines
• Style Guide
• Porting MASTG v1 Tests to v2
• Instructions for new MASTG components

Contributors are expected to understand basic git and GitHub workflows, including forks, branches, commits, and pull requests. The project does not provide training. Pull requests that do not meet these minimum requirements may be closed without review.

[github-actions]

Automated moderation result.

Verdict, ai-slop.
Confidence, 75.

Reasons.
No AI tool disclosure provided
Substantial changes with minimal justification
Generic descriptions of changes
Multiple files modified with little context given

Evidence.

• "Check exactly one option.", AI tool disclosure section left unchecked.
• "This PR closes Mark Xamarin as end of support #3689", The description lacks detailed insight into scope justification.
• "Fixed the markdown linter errors", Indicates low effort and generic maintenance rather than substantial contributions.

Suggested action, Request the author to clarify AI use and justify the extensive changes..

[sushi2k]

@viso27 please stop from opening another Pull Request for the same topic. You have opened now 8 PRs:

Only one PR is allowed per issue. We are expecting that contributors are familiar with git and pull requests and cannot provide support for it, which you also confirmed and can be read here.

I reviewed this PR now and closed #3716.

[sushi2k]

@viso27 please stop from opening another Pull Request for the same topic. You have opened now 8 PRs:

Only one PR is allowed per issue. We are expecting that contributors are familiar with git and pull requests and cannot provide support for it, which you also confirmed and can be read here.

I reviewed this PR now and closed #3716. Please have a look and merge the changes.

[sushi2k]

Thank you for the update @viso27. I merged it now.

Before opening another pull request, please get to know the basics about Pull Requests and also get more familiar with git.