Bug description

Due to a change introduced in Unity 2021.2, several packages (including this one) do not work due to referencing various system libraries. There's a thread on the Unity Forums about this here.

Here's an example of one of the errors (there are hundreds across the source)

Assets/Plugins/MessagePack/MessagePackReader.cs(119,43): error CS0433:
 The type 'ReadOnlySequence<T>' exists in both 
'System.Memory, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
 and 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'

Repro steps

Create a blank project in Unity 2021.2, and add the latest MessagePack-CSharp unitypackage.

Expected behavior

It works.

Actual behavior

Hundreds of compile errors.