docs: Mark Xamarin as end of support

[viso27]

This PR:

• 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.

[github-actions]

Thank you for your contribution! However, this PR does not reference any issues.

To contribute to this project, please:

1. Find an existing issue you'd like to work on, or create a new one
2. Get assigned to the issue
3. Reference the issue in your PR using keywords like "Fixes 0x4 - About Burp Suite #123" or "Closes 0x4 - About Burp Suite #123"

Please reopen this PR once you have linked it to an issue you are assigned to.