Include netstandard2.1 in the set of default C# 8 frameworks (#37365)

src/Compilers/Core/MSBuildTask/Microsoft.CSharp.Core.targets

@@ -3,7 +3,8 @@
 <Project ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="Microsoft.Managed.Core.targets"/>
 
-  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp' OR '$(TargetFrameworkVersion)' != 'v3.0'">
+  <PropertyGroup Condition="('$(TargetFrameworkIdentifier)' != '.NETCoreApp' OR '$(TargetFrameworkVersion)' != 'v3.0') AND
+                            ('$(TargetFrameworkIdentifier)' != '.NETStandard' OR '$(TargetFrameworkVersion)' != 'v2.1')">
     <MaxSupportedLangVersion Condition="'$(MaxSupportedLangVersion)' == ''">7.3</MaxSupportedLangVersion>
     <LangVersion Condition="'$(LangVersion)' == ''">$(MaxSupportedLangVersion)</LangVersion>
   </PropertyGroup>