feat: add net6.0 target framework

Author: phnx47

src/Prometheus.Client.AspNetCore.csproj

@@ -1,8 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Description>ASP.NET Core middleware for the Prometheus.Client</Description>
-    <VersionPrefix>4.5.1</VersionPrefix>
-    <TargetFrameworks>netstandard2.0;netstandard2.1;netcoreapp3.1;net5.0</TargetFrameworks>
+    <VersionPrefix>4.6.0</VersionPrefix>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;netcoreapp3.1;net5.0;net6.0</TargetFrameworks>
     <PackageId>Prometheus.Client.AspNetCore</PackageId>
     <RepositoryUrl>https://github.com/prom-client-net/prom-client-aspnetcore</RepositoryUrl>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
@@ -14,10 +14,10 @@
   <ItemGroup>
     <PackageReference Include="Prometheus.Client" Version="[4.5.3,5.0.0)" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1' OR '$(TargetFramework)' == 'net5.0'">
+  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.1' OR '$(TargetFramework)' == 'net5.0' OR '$(TargetFramework)' == 'net6.0'">
     <FrameworkReference Include="Microsoft.AspNetCore.App" />
   </ItemGroup>
-  <ItemGroup Condition="'$(TargetFramework)' != 'netcoreapp3.1' AND '$(TargetFramework)' != 'net5.0'">
+  <ItemGroup Condition="'$(TargetFramework)' != 'netcoreapp3.1' AND '$(TargetFramework)' != 'net5.0' AND '$(TargetFramework)' != 'net6.0'">
     <PackageReference Include="Microsoft.AspNetCore.Http.Abstractions" Version="2.2.0" />
     <PackageReference Include="System.Text.Encodings.Web" Version="4.5.1" />
   </ItemGroup>