Bump DiffPlex from 1.8.0 to 1.9.0

---
updated-dependencies:
- dependency-name: DiffPlex
  dependency-version: 1.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

src/Directory.Packages.props

@@ -4,7 +4,7 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <ItemGroup>
-    <PackageVersion Include="DiffPlex" Version="1.8.0" />
+    <PackageVersion Include="DiffPlex" Version="1.9.0" />
     <PackageVersion Include="MarkdownSnippets.MsBuild" Version="28.0.0-beta.2" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
     <PackageVersion Include="NUnit" Version="4.4.0" />