[WIP] Migrate from Avalonia.ReactiveUI to ReactiveUI.Avalonia

[Copilot]

Migration from Avalonia.ReactiveUI to ReactiveUI.Avalonia

This PR updates all templates to use the new ReactiveUI.Avalonia package (version 14.7.1) instead of the deprecated Avalonia.ReactiveUI package.

Changes completed:

• Update C# app-mvvm template (package reference + namespace)
• Update C# xplat template - core project (package references)
• Update C# xplat template - Desktop project (namespace)
• Update C# xplat template - Browser project (namespace)
• Update C# xplat template - Android project (namespace)
• Update C# xplat template - iOS project (namespace)
• Update C# xplat template - Directory.Packages.props (package version)
• Update F# app-mvvm template (package reference + namespace)
• Update F# xplat template - core project (package references)
• Update F# xplat template - Desktop project (namespace)
• Update F# xplat template - Browser project (namespace)
• Update F# xplat template - Android project (namespace)
• Update F# xplat template - iOS project (namespace)
• Test template generation and build
• Run code review
• Run security scan

Key changes:

• Package: Avalonia.ReactiveUI → ReactiveUI.Avalonia (version 14.7.1)
• Namespace: using Avalonia.ReactiveUI; → using ReactiveUI.Avalonia;
• F# namespace: open Avalonia.ReactiveUI → open ReactiveUI.Avalonia

Original prompt

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This section details on the original issue you should resolve

<issue_title>Migrate from Avalonia.ReactiveUI to ReactiveUI.Avalonia</issue_title>
<issue_description>See https://github.com/reactiveui/ReactiveUI.Avalonia
New doc https://docs.avaloniaui.net/docs/concepts/reactiveui/</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Migrate from Avalonia.ReactiveUI to ReactiveUI.Avalonia #306

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