PowerShell module does not work in PowerShell 5.1

Brief description of your issue

The Microsoft.WinGet.Source.psd1 module will not load with PowerShell 5.1. It fails with the following error:

Add-Type : Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more
information.
At C:\src\winget-cli-restsource\Tools\PowershellModule\src\Microsoft.WinGet.Source.psm1:24 char:17
+ ...             Add-Type -Path "$PSScriptRoot\Library\HelperLib\x64\Micro ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Add-Type], ReflectionTypeLoadException
    + FullyQualifiedErrorId : System.Reflection.ReflectionTypeLoadException,Microsoft.PowerShell.Commands.AddTypeComma
   nd

LoaderExceptions contains the following missing dependencies:

Could not load file or assembly 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or
one of its dependencies. The system cannot find the file specified.
Could not load file or assembly 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or
one of its dependencies. The system cannot find the file specified.
Could not load file or assembly 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or
one of its dependencies. The system cannot find the file specified.

The README.md does not make any mention that PowerShell 5.1 is not supported and that you need a newer version of PowerShell. (It loaded for me fine in PowerShell 7, after working around #172. :))

This is not called out in the documentation as a prerequisite.