Bump DiffPlex from 1.4.1 to 1.4.4 in /Rx.NET/Source

Bumps [DiffPlex](https://github.com/mmanela/diffplex) from 1.4.1 to 1.4.4.
- [Release notes](https://github.com/mmanela/diffplex/releases)
- [Commits](https://github.com/mmanela/diffplex/commits/v1.4.4)

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

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

@@ -39,7 +39,7 @@
     <PackageReference Include="xunit" Version="2.4.0-beta.2.build4010" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0-beta.2.build4010" />
     <PackageReference Include="ApprovalTests" Version="3.0.14" />
-    <PackageReference Include="DiffPlex" Version="1.4.1" />
+    <PackageReference Include="DiffPlex" Version="1.4.4" />
     <PackageReference Include="PublicApiGenerator" Version="7.0.1" />
   </ItemGroup>