Avoid spurious references to Microsoft.Bcl.AsyncInterfaces (#1719)

Author: WeihanLi

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -26,7 +26,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Condition="'$(TargetFramework)' != 'netcoreapp3.1' and '$(TargetFramework)' != 'netstandard2.1' " Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
+    <PackageReference Condition="'$(TargetFramework)' == 'net48' or '$(TargetFramework)' == 'netstandard2.0' " Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
     <ReferenceAssemblyProjectReference Include="..\refs\System.Linq.Async.Ref\System.Linq.Async.Ref.csproj" />
     <ProjectReference Include="..\System.Linq.Async.SourceGenerator\System.Linq.Async.SourceGenerator.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" />
   </ItemGroup>