Update targets to .NET 5.0, NET Framework 4.72 and NET Framework 4.8.

[Turochamp]

Created a new version with updated targets, so that package works well with latest versions of .NET SDK.

This PR is a draft with two open questions:

1. Which targets to include? Suggest that package includes the versions supported by dotnet 3.1. Reason is a bug that always restores the latest fsharp.core version. More info about fsharp.core bug: https://github.com/fsprojects/Paket/issues/3769

2. How to automate testing? The run all test command in appveyor-test.ps1 (dotnet msbuild /m Tests/tests.proj) does not work anymore, instead I've used dotnet test Foq.sln.