Housekeeping: Retire Xamarin due to end of support (#3797)

<!-- Please be sure to read the
[Contribute](https://github.com/reactiveui/reactiveui#contribute)
section of the README -->

**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Update

**What is the current behavior?**
<!-- You can also link to an open issue here. -->

Support for Xamarin is included

**What is the new behavior?**
<!-- If this is a feature change -->

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**:

README.md

@@ -145,37 +145,37 @@ See [Contribution Guidelines](https://www.reactiveui.net/contribute/) for furthe
 <table>
   <tbody>
     <tr>
-      <td align="center" valign="top">
+      <td align="center" valign="top" width="105">
         <img width="100" height="100" src="https://github.com/glennawatson.png?s=150">
         <br>
         <a href="https://github.com/glennawatson">Glenn Watson</a>
         <p>Melbourne, Australia</p>
       </td>
-      <td align="center" valign="top">
+      <td align="center" valign="top" width="105">
+        <img width="100" height="100" src="https://github.com/chrispulman.png?s=150">
+        <br>
+        <a href="https://github.com/chrispulman">Chris Pulman</a>
+        <p>United Kingdom</p>
+      </td>
+      <td align="center" valign="top" width="105">
         <img width="100" height="100" src="https://github.com/rlittlesii.png?s=150">
         <br>
         <a href="https://github.com/rlittlesii">Rodney Littles II</a>
         <p>Texas, USA</p>
       </td>
-      <td align="center" valign="top">
-        <img width="100" height="100" src="https://github.com/worldbeater.png?s=150">
-        <br>
-        <a href="https://github.com/worldbeater">Artyom Gorchakov</a>
-        <p>Moscow, Russia</p>
-      </td>
     </tr>
     <tr>
-      <td align="center" valign="top">
+      <td align="center" valign="top" width="105">
         <img width="100" height="100" src="https://github.com/cabauman.png?s=150">
         <br>
         <a href="https://github.com/cabauman">Colt Bauman</a>
         <p>South Korea</p>
       </td>
-      <td align="center" valign="top">
-        <img width="100" height="100" src="https://github.com/chrispulman.png?s=150">
+      <td align="center" valign="top" width="105">
+        <img width="100" height="100" src="https://github.com/worldbeater.png?s=150">
         <br>
-        <a href="https://github.com/chrispulman">Chris Pulman</a>
-        <p>United Kingdom</p>
+        <a href="https://github.com/worldbeater">Artyom Gorchakov</a>
+        <p>Moscow, Russia</p>
       </td>
     </tr>
   </tbody>

src/Directory.Packages.props

@@ -24,6 +24,7 @@
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
     <PackageVersion Include="Microsoft.Windows.SDK.BuildTools" Version="10.0.22621.3233" />
     <PackageVersion Include="Microsoft.Xaml.Behaviors.Wpf" Version="1.1.77" />
+    <PackageVersion Include="Mocks.Maui" Version="1.1.4" />
     <PackageVersion Include="Nerdbank.GitVersioning" Version="3.6.133" />
     <PackageVersion Include="PublicApiGenerator" Version="11.1.0" />
     <PackageVersion Include="Reactive.Wasm" Version="2.1.2" />
@@ -95,4 +96,4 @@
     <PackageVersion Include="System.Text.Encoding.Extensions" Version="4.3.0" />
     <PackageVersion Include="System.Threading" Version="4.3.0" />
   </ItemGroup>
-</Project>
+</Project>

src/Directory.build.props

@@ -32,6 +32,9 @@
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <IncludePackageReferencesDuringMarkupCompilation>true</IncludePackageReferencesDuringMarkupCompilation>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <WindowsTargetFrameworks>net462;net472;net6.0-windows10.0.17763.0;net7.0-windows10.0.17763.0;net8.0-windows10.0.17763.0;net6.0-windows10.0.19041.0;net7.0-windows10.0.19041.0;net8.0-windows10.0.19041.0</WindowsTargetFrameworks>
+    <MobileTargetFrameworks>net7.0-android;net7.0-ios;net7.0-tvos;net7.0-macos;net7.0-maccatalyst;net8.0-android;net8.0-ios;net8.0-tvos;net8.0-macos;net8.0-maccatalyst</MobileTargetFrameworks>
+    <BaseTargetFrameworks>netstandard2.0;net6.0;net7.0;net8.0</BaseTargetFrameworks>
   </PropertyGroup>
   <!--<PropertyGroup Condition="$(IsTestProject) != 'true'">
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

src/Directory.build.targets

@@ -13,9 +13,6 @@
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net4'))">
     <DefineConstants>$(DefineConstants);NET_461;XAML</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.iOS'))">
-    <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;IOS</DefineConstants>
-  </PropertyGroup>
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net6.0-ios'))">
     <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;IOS</DefineConstants>
   </PropertyGroup>
@@ -34,9 +31,6 @@
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net8.0-maccatalyst'))">
     <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;IOS</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.Mac'))">
-    <DefineConstants>$(DefineConstants);MONO;COCOA;MAC</DefineConstants>
-  </PropertyGroup>
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net6.0-macos'))">
     <DefineConstants>$(DefineConstants);MONO;COCOA;MAC</DefineConstants>
   </PropertyGroup>
@@ -46,9 +40,6 @@
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net8.0-macos'))">
     <DefineConstants>$(DefineConstants);MONO;COCOA;MAC</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.TVOS'))">
-    <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;TVOS</DefineConstants>
-  </PropertyGroup>
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net6.0-tvos'))">
     <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;TVOS</DefineConstants>
   </PropertyGroup>
@@ -58,13 +49,6 @@
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net8.0-tvos'))">
     <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;TVOS</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('Xamarin.WatchOS'))">
-    <DefineConstants>$(DefineConstants);MONO;UIKIT;COCOA;WATCHOS</DefineConstants>
-  </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('MonoAndroid'))">
-    <DefineConstants>$(DefineConstants);MONO;ANDROID</DefineConstants>
-    <DesignTimeBuild>false</DesignTimeBuild>
-  </PropertyGroup>
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net6.0-android'))">
     <DefineConstants>$(DefineConstants);MONO;ANDROID</DefineConstants>
   </PropertyGroup>
@@ -74,9 +58,6 @@
   <PropertyGroup Condition="$(TargetFramework.StartsWith('net8.0-android'))">
     <DefineConstants>$(DefineConstants);MONO;ANDROID</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="$(TargetFramework.StartsWith('tizen'))">
-    <DefineConstants>$(DefineConstants);TIZEN</DefineConstants>
-  </PropertyGroup>
 
   <Target Name="ValidateNugetProperties" Condition="!$(IsTestProject) or $(IsTestProject) == ''" BeforeTargets="Compile">
     <Error Condition="$(PackageDescription) == '' or $(PackageDescription) == $(DefaultPackageDescription)" Text="The Nuget PackageDescription property needs to be set for the project. Currently : '$(PackageDescription)'" />