test: update target frameworks

Author: phnx47

tests/Prometheus.Client.MetricServer.Tests.csproj

@@ -1,15 +1,16 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp3.1;net6.0;net7.0</TargetFrameworks>
-    <InvariantGlobalization>true</InvariantGlobalization>
+    <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
     <NoWarn>$(NoWarn);CS0618</NoWarn>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="xunit" Version="2.9.0" />
-    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1'" Include="xunit.runner.visualstudio" Version="2.8.2" />
-    <PackageReference Condition="'$(TargetFramework)' == 'netcoreapp3.1'" Include="xunit.runner.visualstudio" Version="2.4.*" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
     <PackageReference Include="coverlet.collector" Version="6.0.2">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>