chore(deps): update xunit-dotnet monorepo to v2.8.0

Author: renovate[bot]

tests/Prometheus.Client.MetricServer.Tests.csproj

@@ -7,8 +7,8 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
-    <PackageReference Include="xunit" Version="2.7.1" />
-    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1'" Include="xunit.runner.visualstudio" Version="2.5.8" />
+    <PackageReference Include="xunit" Version="2.8.0" />
+    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1'" Include="xunit.runner.visualstudio" Version="2.8.0" />
     <PackageReference Condition="'$(TargetFramework)' == 'netcoreapp3.1'" Include="xunit.runner.visualstudio" Version="2.4.*" />
     <PackageReference Include="coverlet.collector" Version="6.0.2">
       <PrivateAssets>all</PrivateAssets>