Bump xunit to 2.8.0

Author: AArnott

Directory.Packages.props

@@ -7,8 +7,8 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
-    <PackageVersion Include="xunit.runner.visualstudio" Version="2.5.7" />
-    <PackageVersion Include="xunit" Version="2.7.1" />
+    <PackageVersion Include="xunit.runner.visualstudio" Version="2.8.0" />
+    <PackageVersion Include="xunit" Version="2.8.0" />
   </ItemGroup>
   <ItemGroup>
     <GlobalPackageReference Include="CSharpIsNullAnalyzer" Version="0.1.495" />