Use .NET Core 3.1

Author: FlorianRappl

src/AngleSharp.Io.Tests/AngleSharp.Io.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>netcoreapp3.1</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>Key.snk</AssemblyOriginatorKeyFile>
     <IsPackable>false</IsPackable>
@@ -18,15 +18,15 @@
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="GitHubActionsTestLogger" Version="1.2.0" />
-    <PackageReference Include="NUnit" Version="3.13.1" />
+    <PackageReference Include="NUnit" Version="3.13.2" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.17.0" />
     <PackageReference Include="FluentAssertions" Version="5.10.3" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.4" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.10.0" />
   </ItemGroup>
 
   <ItemGroup>
     <None Update="TestContent.txt">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
     </None>
   </ItemGroup>
-</Project>
+</Project>