Update MAUI model tester tool to .net8 (#19907)

### Description
<!-- Describe your changes. -->
Update to .net8. Didn't want to build with the latest VS2022 using net6
(which was EOL last year).



### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

csharp/tools/MauiModelTester/MauiModelTester.csproj

@@ -1,8 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>net6.0-android;net6.0-ios</TargetFrameworks>
-		<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net6.0-windows10.0.19041.0</TargetFrameworks>
+		<TargetFrameworks>net8.0-ios;net8.0-android34.0</TargetFrameworks>
+		<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net8.0-windows10.0.19041.0</TargetFrameworks>
 		<OutputType>Exe</OutputType>
 		<RootNamespace>MauiModelTester</RootNamespace>
 		<UseMaui>true</UseMaui>
@@ -21,7 +21,7 @@
 		<ApplicationVersion>1</ApplicationVersion>
 
 		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'ios'">12.0</SupportedOSPlatformVersion>
-		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'android'">21.0</SupportedOSPlatformVersion>
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'android'">29.0</SupportedOSPlatformVersion>
 		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</SupportedOSPlatformVersion>
 		<TargetPlatformMinVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</TargetPlatformMinVersion>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>

csharp/tools/MauiModelTester/Platforms/Android/AndroidManifest.xml

@@ -4,5 +4,5 @@
 	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
 	<uses-permission android:name="android.permission.INTERNET" />
 	<uses-permission android:name="android.permission.DIAGNOSTIC" />
-	<uses-sdk android:minSdkVersion="21" android:targetSdkVersion="31" />
+	<uses-sdk android:minSdkVersion="29" android:targetSdkVersion="34" />
 </manifest>