Dropped .NET standard 1.3

Author: FlorianRappl

src/AngleSharp.Xml/AngleSharp.Xml.csproj

@@ -2,8 +2,8 @@
   <PropertyGroup>
     <AssemblyName>AngleSharp.Xml</AssemblyName>
     <RootNamespace>AngleSharp.Xml</RootNamespace>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard1.3;netstandard2.0</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">netstandard1.3;netstandard2.0;net46;net461;net472</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">netstandard2.0</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">netstandard2.0;net46;net461;net472</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>Key.snk</AssemblyOriginatorKeyFile>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>