Use Xunit runner that is compatible with .NET 5

Using the 2.4.1 version of the runner fails with "Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again." when executing `dotnet test` on .NET 5 project. 2,4,3 was specifically released to address this. See xunit/visualstudio.xunit#229.

src/Microsoft.DotNet.Arcade.Sdk/tools/DefaultVersions.props

@@ -86,7 +86,7 @@
     <MicrosoftTestPlatformVersion Condition="'$(MicrosoftTestPlatformVersion)' == ''">16.5.0</MicrosoftTestPlatformVersion>
     <XUnitVersion Condition="'$(XUnitVersion)' == ''">2.4.1</XUnitVersion>
     <XUnitRunnerConsoleVersion Condition="'$(XUnitRunnerConsoleVersion)' == ''">$(XUnitVersion)</XUnitRunnerConsoleVersion>
-    <XUnitRunnerVisualStudioVersion Condition="'$(XUnitRunnerVisualStudioVersion)' == ''">$(XUnitVersion)</XUnitRunnerVisualStudioVersion>
+    <XUnitRunnerVisualStudioVersion Condition="'$(XUnitRunnerVisualStudioVersion)' == ''">2.4.3</XUnitRunnerVisualStudioVersion>
     <MSTestVersion Condition="'$(MSTestVersion)' == ''">2.0.0</MSTestVersion>
     <MSTestTestAdapterVersion Condition="'$(MSTestTestAdapterVersion)' == ''">$(MSTestVersion)</MSTestTestAdapterVersion>
     <MSTestTestFrameworkVersion Condition="'$(MSTestTestFrameworkVersion)' == ''">$(MSTestVersion)</MSTestTestFrameworkVersion>