ci: roll test host forward so net6.0 tests run on current runners

build-prism-core and build-prism-wpf built cleanly but failed at "Run Tests":
the test projects target net6.0, whose runtime is EOL and no longer present on
the runner images, so the test host aborted with "You must install or update
.NET to run this application" before any test executed.

Set RollForward=Major for test projects so the host rolls net6.0 forward to the
newest installed major runtime. Runtime selection only - the test assemblies are
still built against net6.0. Verified locally: Prism.Core.Tests passes 277/277 and
the property lands in the test runtimeconfig the host reads.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dansiegel

Directory.Build.props

@@ -138,6 +138,17 @@
     <None Include="build\Package.targets" Pack="true" PackagePath="buildTransitive\$(PackageId).targets" Condition="exists('build\Package.targets')"/>
   </ItemGroup>
 
+  <!--
+    The test projects target net6.0, which is end-of-life and no longer present on the CI
+    runner images (only newer runtimes are installed). Roll the test host forward to the
+    newest installed major runtime so the tests can execute; without this the testhost aborts
+    with "You must install or update .NET to run this application". This affects runtime
+    selection only - the test assemblies are still built against net6.0.
+  -->
+  <PropertyGroup Condition=" $(IsTestProject) ">
+    <RollForward>Major</RollForward>
+  </PropertyGroup>
+
   <ItemGroup Condition=" $(IsTestProject) ">
     <None Include="$(MSBuildThisFileDirectory)xunit.runner.json"
           CopyToOutputDirectory="PreserveNewest" />