Update AndroidX project to use MauiAndroidTargets variable and remove net8.0-android

- Updated ReactiveUI.Validation.AndroidX.csproj to use $(MauiAndroidTargets) variable
- Removed deprecated net8.0-android target (Microsoft deprecated)
- Now targets net9.0-android and net10.0-android only
- Kept ReactiveUI packages at 21.0.1 (22.0.1 has breaking changes)
- AndroidX project builds successfully
- All tests passing

Co-authored-by: glennawatson <5834289+glennawatson@users.noreply.github.com>

Author: Copilot

src/ReactiveUI.Validation.AndroidX/ReactiveUI.Validation.AndroidX.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net8.0-android;net9.0-android</TargetFrameworks>
+    <TargetFrameworks>$(MauiAndroidTargets)</TargetFrameworks>
     <PackageDescription>Provides ReactiveUI.Validation extensions for the AndroidX Library</PackageDescription>
     <PackageId>ReactiveUI.Validation.AndroidX</PackageId>
     <NoWarn>$(NoWarn);CS1591</NoWarn>