Update projects to netcoreapp3.1 and netstandard2.1

[niccou]

I updated the projects to netstandard2.1 and netcoreapp3.1 (LTS) from the initialization to the end.

I also added a tool-manifest to install paket

I updated all paket.references to latest version

Modified files (out of the rebase action) :

• .config/dotnet-tools.json(added)

• paket.dependencies

• paket.lock

• Mixter/Mixter.fsproj

• Mixter.Tests/Mixter.Tests.fsproj

• Mixter.Tests/Mixter.Tests.fs(deleted)

[fpellet]

Thank you. I will rename this branch to fsharp/v2.1