Update target framework to net8.0

Author: phnx47

src/Prometheus.Client.HttpClient/Prometheus.Client.HttpClient.csproj

@@ -1,10 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Prometheus.Client" Version="6.1.0" />
-    <PackageReference Include="Microsoft.Extensions.Http" Version="6.0.1" />
+    <PackageReference Include="Microsoft.Extensions.Http" Version="8.0.1" />
   </ItemGroup>
   <ItemGroup>
     <InternalsVisibleTo Include="$(MSBuildProjectName).Tests, PublicKey=002400000480000094000000060200000024000052534131000400000100010005cd7fff58a5f07dd84ba50975028ab8f3e6d86aa42579628f27012bdcb335ab71f06ec08a39d4308e0be6d63507b9afdd4763e7625251a68912bfadb5b411627a397e6f70a20eb28f5c841e8e8354fc19a1d538ee079575a59678edc98597f98f659b697c46d5f1ca1f1b0b8be057c9eb0a1496a2da78074760026ee98a3ceb" />

tests/Prometheus.Client.HttpClient.Tests/Prometheus.Client.HttpClient.Tests.csproj

@@ -1,15 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Fennel" Version="0.3.0" />
     <PackageReference Include="FluentAssertions" Version="8.8.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
-    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.v3" Version="3.2.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>