Housekeeping: Retire Xamarin due to end of support

[ChrisPulman]

What kind of change does this PR introduce?

Update

What is the current behavior?

Support for Xamarin is included

What is the new behavior?

Xamarin Support is retired inline with Microsoft.
https://dotnet.microsoft.com/en-us/platform/support/policy/xamarin

What might this PR break?

Users wishing to continue using Xamarin will need to continue using release 19.6.12 of ReactiveUI, ReactiveUI.XamarinForms , ReactiveUI.AndroidX and ReactiveUI.AndroidSupport

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[anaisbetts]

RIP tha Dream 😭

[ChrisPulman]

RIP tha Dream 😭

I know, 14 years, but its more of a migration to a new name than a complete loss, without your dream this project and its related projects would have left the world struggling using imperative code bases. Thank you

[glennawatson]

I'll leave it up to you to decide @ChrisPulman when to merge it in -- I guess the support drops on the 1st May US time so we should do a release after that date.

[ChrisPulman]

I'll leave it up to you to decide @ChrisPulman -- I guess the support drops on the 1st May US time so we should do a release after that date.

This is the intended release date in my mind, thank you

[github-actions]

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