update tfm to net8.0

Author: phnx47

src/Prometheus.Client.AspNetCore.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <Description>ASP.NET Core middleware for the Prometheus.Client</Description>
-    <TargetFrameworks>netcoreapp3.1;net5.0</TargetFrameworks>
+    <Description>ASP.NET Core middleware for Prometheus.Client</Description>
+    <TargetFramework>net8.0</TargetFramework>
     <RepositoryUrl>https://github.com/prom-client-net/prom-client-aspnetcore</RepositoryUrl>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>

tests/Prometheus.Client.AspNetCore.Tests.csproj

@@ -1,12 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
+    <TargetFramework>net8.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <NoWarn>$(NoWarn);CS0618</NoWarn>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
-    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.v3" Version="2.0.2 " />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>