chore(deps): update dependency diffengine to 15.9.0 (#737)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

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

@@ -22,7 +22,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="DiffEngine" Version="15.6.0" />
+    <PackageReference Include="DiffEngine" Version="15.9.0" />
     <PackageReference Include="JetBrains.DotMemoryUnit" Version="3.2.20220510" />
     <PackageReference Include="FluentAssertions" Version="8.0.1" />
   </ItemGroup>