Make JsonPatch netstandard2.0

Fixes https://github.com/aspnet/AspNetCore/issues/9812

src/Features/JsonPatch/ref/Microsoft.AspNetCore.JsonPatch.csproj

@@ -1,10 +1,10 @@
 <!-- This file is automatically generated. -->
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp3.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
   </PropertyGroup>
-  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0'">
-    <Compile Include="Microsoft.AspNetCore.JsonPatch.netcoreapp3.0.cs" />
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <Compile Include="Microsoft.AspNetCore.JsonPatch.netstandard2.0.cs" />
     <Reference Include="Microsoft.CSharp"  />
     <Reference Include="Newtonsoft.Json"  />
   </ItemGroup>

src/Features/JsonPatch/src/Microsoft.AspNetCore.JsonPatch.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <Description>ASP.NET Core support for JSON PATCH.</Description>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;json;jsonpatch</PackageTags>