Remove DiffEngine and update Verify.NUnit to 31.0.1

Co-authored-by: glennawatson <[email protected]>

Author: Copilot

src/ReactiveUI.Validation.Tests/ReactiveUI.Validation.Tests.csproj

@@ -16,12 +16,11 @@
     <PackageReference Include="NUnit.Analyzers" Version="4.10.0" />
     <PackageReference Include="Microsoft.Reactive.Testing" Version="6.0.2" />
     <PackageReference Include="PublicApiGenerator" Version="11.4.6" />
-    <PackageReference Include="Verify.NUnit" Version="30.*" />
+    <PackageReference Include="Verify.NUnit" Version="31.0.1" />
     <PackageReference Include="coverlet.msbuild" Version="6.0.4">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="DiffEngine" Version="16.*" />
     <PackageReference Include="JetBrains.DotMemoryUnit" Version="3.2.20220510" />
   </ItemGroup>