Update xunit

- Fix some warnings it created

Author: andrewlock

tracer/test/Datadog.Trace.Tools.Runner.Tests/Datadog.Trace.Tools.Runner.Tests.csproj

@@ -31,7 +31,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Verify.Xunit" Version="20.8.2" />
+    <PackageReference Include="Verify.Xunit" Version="14.13.1" />
     <PackageReference Include="DiffPlex" Version="1.7.2" />
   </ItemGroup>