[C#] Enable dual build for netstandard2.0 & netstandard2.1

Author: pierre-irrmann

csharp/sbe-dll/sbe-dll.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
     <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
     <AssemblyName>SBE</AssemblyName>
     <RootNamespace>Org.SbeTool.Sbe.Dll</RootNamespace>
@@ -38,9 +38,13 @@
     </None>
   </ItemGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
-    <PackageReference Include="System.Memory" Version="4.5.3" />
-  </ItemGroup>
+  <Choose>
+    <When Condition="'$(TargetFramework)' == 'netstandard2.0'">
+      <ItemGroup>
+        <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
+        <PackageReference Include="System.Memory" Version="4.5.3" />
+      </ItemGroup>
+    </When>
+  </Choose>
 
 </Project>