"Warning: Invalid cref value" when targeting .NET Standard 2.1

[Overlong]

Operating System: Windows

DocFX Version Used: 2.56.2

Template used: default

Steps to Reproduce:

1. Create a new .NET Standard Class Library in Visual Studio 2019 Community Edition.
2. Add a class with XML documentation comments referring to a standard library class:

using System;

namespace DocFxIssue
{
    /// <summary>
    /// This summary refers to the <see cref="Math"/> class.
    /// </summary>
    public class MyClass
    {
    }
}

3. Install NuGet package docfx.console (version 2.56.2) for the project.
4. Change target framework from .NET Standard 2.0 to .NET Standard 2.1.
5. Build project.

Expected Behavior:

• No warnings when building the project.
• A reference to the Math class in the generated reference page for the MyClass class:

  This summary refers to the System.Math class.

Actual Behavior:

• Get warning

  Warning:[MetadataCommand.ExtractMetadata]Invalid cref value "!:Math" found in triple-slash-comments for MyClass defined in MyClass.cs Line 7, ignored.

when building the project.

• Get no reference to the Math class in the generated reference page for the MyClass class but rather just

  This summary refers to the class.

Remarks:

• References in seealso and exception tags do not work either.
• If step 4 is omitted, the expected behavior is observed.
• Using the fully qualified type name System.Math as cref value does not work.
• References to other classes in the project do work as expected.

Related issues: The same warning (but probably for other reasons) is observed in issues #628, #1706, #4631, and #5112.