chore(deps): update dependency diffengine to 15.6.0

Author: renovate[bot]

src/Directory.Packages.props

@@ -14,7 +14,7 @@
     <PackageVersion Include="Citrus.Avalonia" Version="2.1.1" />
     <PackageVersion Include="MessageBox.Avalonia" Version="3.2.0" />
     <PackageVersion Include="coverlet.msbuild" Version="6.0.2" />
-    <PackageVersion Include="DiffEngine" Version="15.5.3" />
+    <PackageVersion Include="DiffEngine" Version="15.6.0" />
     <PackageVersion Include="FluentAssertions" Version="6.12.2" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="6.0.1" />