build(deps): bump DiffEngine from 8.3.0 to 8.4.0 in /src (#384)

Bumps [DiffEngine](https://github.com/VerifyTests/DiffEngine) from 8.3.0 to 8.4.0.
- [Release notes](https://github.com/VerifyTests/DiffEngine/releases)
- [Commits](https://github.com/VerifyTests/DiffEngine/commits)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

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

@@ -23,7 +23,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="DiffEngine" Version="8.3.0" />    
+    <PackageReference Include="DiffEngine" Version="8.4.0" />    
   </ItemGroup>
 
   <ItemGroup>