Update netcoreapp2.1 -> net5.0

Author: iamcarbon

src/AngleSharp.Css.Tests/AngleSharp.Css.Tests.csproj

@@ -1,10 +1,10 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>net5.0</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>Key.snk</AssemblyOriginatorKeyFile>
     <IsPackable>false</IsPackable>
-    <LangVersion>9.0</LangVersion>
+    <LangVersion>9</LangVersion>
     <AssemblyName>AngleSharp.Css.Tests</AssemblyName>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies> <!-- https://github.com/Tyrrrz/GitHubActionsTestLogger/issues/5 -->
   </PropertyGroup>

src/AngleSharp.Performance.Common/AngleSharp.Performance.Common.csproj

@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>7.1</LangVersion>
+    <LangVersion>9</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>

src/AngleSharp.Performance.Css/AngleSharp.Performance.Css.csproj

@@ -1,11 +1,11 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
     <ApplicationIcon />
     <OutputType>Exe</OutputType>
     <StartupObject />
     <AutoGenerateBindingRedirects>false</AutoGenerateBindingRedirects>
-    <LangVersion>7.1</LangVersion>
+    <LangVersion>9</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(TargetFramework)|$(Platform)'=='Debug|netstandard2.0|AnyCPU'">
     <PlatformTarget>AnyCPU</PlatformTarget>

src/AngleSharp.Performance.Utilities/AngleSharp.Performance.Utilities.csproj

@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
-    <LangVersion>7.1</LangVersion>
+    <LangVersion>9</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>