#2401 Add net core 3 so package dependency can be removed

Author: thompson-tomo

src/Grpc.HealthCheck/Grpc.HealthCheck.csproj

@@ -5,7 +5,7 @@
 
     <IsGrpcPublishedPackage>true</IsGrpcPublishedPackage>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
-    <TargetFrameworks>net462;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net462;netstandard2.0;netcoreapp3.0</TargetFrameworks>
     <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
 
@@ -19,11 +19,15 @@
 
   <ItemGroup>
     <PackageReference Include="Google.Protobuf" />
-    <PackageReference Include="System.Threading.Channels" />
 
     <!-- TODO(jtattermusch): add reference to Grpc.Tools and use it for generating the Health.cs and HealthGrpc.cs sources -->
   </ItemGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net462' or '$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="System.Threading.Channels" />
+
+  </ItemGroup>
+
   <ItemGroup Condition=" '$(TargetFramework)' == 'net462' ">
     <Reference Include="System" />
     <Reference Include="Microsoft.CSharp" />