switch GitVersionTask.MsBuild to netstandard 2.0

This reverts commit 97487a2.

Author: arturcic

src/GitVersionTask.MsBuild/GitVersionTask.MsBuild.csproj

@@ -1,12 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net472;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>
+        <TargetFrameworks>netstandard2.0</TargetFrameworks>
         <LangVersion>8.0</LangVersion>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     </PropertyGroup>
 
     <ItemGroup>
+        <PackageReference Include="System.Runtime.Loader" Version="4.3.0" />
         <PackageReference Include="Microsoft.Build" Version="16.4.0" />
         <PackageReference Include="Microsoft.Build.Tasks.Core" Version="16.4.0" />
     </ItemGroup>