Bump Verify.Xunit from 1.34.2 to 6.21.0 in /Rx.NET/Source

Bumps [Verify.Xunit](https://github.com/VerifyTests/Verify) from 1.34.2 to 6.21.0.
- [Release notes](https://github.com/VerifyTests/Verify/releases)
- [Commits](https://github.com/VerifyTests/Verify/compare/1.34.2...6.21.0)

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

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

@@ -26,7 +26,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.0" />
-    <PackageReference Include="Verify.Xunit" Version="1.34.2" />
+    <PackageReference Include="Verify.Xunit" Version="6.21.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
     <PackageReference Include="DiffPlex" Version="1.6.3" />