[Xamarin.Android.Build.Tasks] introduce Eol.targets for .NET 6

Fixes: #8003

Add a new `Eol.targets` file to be imported for `net6.0-android`
projects. It contains the necessary/minimal settings to emit the build
error:

    Microsoft.NET.EolTargetFrameworks.targets(35,5): error NETSDK1202:
    The workload 'android' is out of support and will not receive security updates in the future. Please refer to https://aka.ms/maui-support-policy for more information about the support policy.

We will probably need the latest .NET SDK to flow to us, in order to
write a test. I could only test locally by editing `.targets` files in
the .NET SDK.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/Sdk/Eol.targets

@@ -0,0 +1,44 @@
+<!--
+***********************************************************************************************
+Eol.targets
+
+Imported only by .NET 6 EOL projects, contains settings to force the error:
+
+error NETSDK1202: The workload 'android' is out of support and will not receive security updates in the future. Please refer to https://aka.ms/maui-support-policy for more information about the support policy.
+
+Things to note:
+
+* $(WarningsAsErrors) includes NETSDK1202
+* Force $(TargetPlatformVersion) to just be 21.0, to make it past early error messages
+* _ClearMissingWorkloads prevents undesired extra error messages
+
+***********************************************************************************************
+-->
+<Project InitialTargets="_ClearMissingWorkloads">
+  <PropertyGroup>
+    <!-- Force NETSDK1202 to be an error -->
+    <WarningsAsErrors>$(WarningsAsErrors);NETSDK1202</WarningsAsErrors>
+    <!--
+      Along with @(SdkSupportedTargetPlatformVersion), prevents the error:
+      Microsoft.NET.TargetFrameworkInference.targets(229,5):
+      error NETSDK1140: 31.0 is not a valid TargetPlatformVersion for android. Valid versions include:
+    -->
+    <TargetPlatformSupported>true</TargetPlatformSupported>
+    <TargetPlatformVersion>21.0</TargetPlatformVersion>
+  </PropertyGroup>
+  <ItemGroup>
+    <SdkSupportedTargetPlatformVersion Include="21.0" />
+    <EolWorkload Include="android" Url="https://aka.ms/maui-support-policy" />
+  </ItemGroup>
+  <Target Name="_ClearMissingWorkloads">
+    <!--
+      Prevents the error:
+      Microsoft.NET.Sdk.ImportWorkloads.targets(38,5): error NETSDK1147:
+      To build this project, the following workloads must be installed: wasm-tools-net6
+      To install these workloads, run the following command: dotnet workload restore
+    -->
+    <ItemGroup>
+      <MissingWorkloadPack Remove="@(MissingWorkloadPack)" />
+    </ItemGroup>
+  </Target>
+</Project>

src/Xamarin.Android.Build.Tasks/Microsoft.NET.Sdk.Android/WorkloadManifest.in.targets

@@ -4,7 +4,7 @@
         Condition=" $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '8.0')) " />
     <Import Project="Sdk.targets" Sdk="Microsoft.Android.Sdk.net7"
         Condition=" $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '7.0')) " />
-    <Import Project="Sdk.targets" Sdk="Microsoft.Android.Sdk.net6"
+    <Import Project="Eol.targets" Sdk="Microsoft.Android.Sdk.net8"
         Condition=" $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '6.0')) " />
   </ImportGroup>
 
@@ -16,7 +16,7 @@
     />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0')) ">
+  <ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '7.0')) ">
     <SdkSupportedTargetPlatformIdentifier Include="android" DisplayName="Android" />
   </ItemGroup>
 </Project>