Bump DiffPlex from 1.6.0 to 1.6.1 in /Rx.NET/Source

Bumps [DiffPlex](https://github.com/mmanela/diffplex) from 1.6.0 to 1.6.1.
- [Release notes](https://github.com/mmanela/diffplex/releases)
- [Commits](https://github.com/mmanela/diffplex/commits)

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

Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Tests.System.Reactive.ApiApprovals.csproj

@@ -29,7 +29,7 @@
     <PackageReference Include="Verify.Xunit" Version="1.27.3" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
-    <PackageReference Include="DiffPlex" Version="1.6.0" />
+    <PackageReference Include="DiffPlex" Version="1.6.1" />
     <PackageReference Include="PublicApiGenerator" Version="10.0.2" />
   </ItemGroup>