feat: support netstandard2.0 (#44)

Author: catcherwong

src/Dtmcli/Dtmcli.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
 	<PropertyGroup>
-		<TargetFrameworks>netstandard2.1;net5.0;net6.0</TargetFrameworks>
+		<TargetFrameworks>netstandard2.0;netstandard2.1;net5.0;net6.0</TargetFrameworks>
 		<PackageProjectUrl>https://github.com/dtm-labs/dtmcli-csharp</PackageProjectUrl>
 		<RepositoryUrl>https://github.com/dtm-labs/dtmcli-csharp</RepositoryUrl>
 		<AssemblyName>Dtmcli</AssemblyName>