Update target framework to .NET Standard 2.1 and .NET Core 3.1

Source/Managed/ENet-CSharp.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Library</OutputType>
-    <TargetFrameworks>netstandard2.0;netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.1;netcoreapp3.1</TargetFrameworks>
     <Platforms>x64</Platforms>
     <RootNamespace>ENet</RootNamespace>
     <LangVersion>3</LangVersion>