build(deps): bump DiffEngine from 13.0.2 to 15.1.1 in /src

Bumps [DiffEngine](https://github.com/VerifyTests/DiffEngine) from 13.0.2 to 15.1.1.
- [Release notes](https://github.com/VerifyTests/DiffEngine/releases)
- [Commits](VerifyTests/DiffEngine@13.0.2...15.1.1)

---
updated-dependencies:
- dependency-name: DiffEngine
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

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="15.0.1" />    
+    <PackageReference Include="DiffEngine" Version="15.1.1" />    
   </ItemGroup>
 
   <ItemGroup>