Make public api and tests the same for all TFMs

[Romfos]

Changes:

• Make public api and tests the same for all TFMs
• Run the same set of tests for all platforms
• Update System.Threading.Tasks.Extensions dependency

Motivation: Some public api is not available for for .NET Standard in current implementation. Problem is that ValueTask is a core type in modern dotnet ecosystem. A lot of popular libraries using it. Make sense to remove this gap and make public api the same for all platforms

Why System.Threading.Tasks.Extensions was updated? Old version doesn't have non-generic ValueTask type