Remove netcoreapp2.1 support #1067
Description
Like #1031, but for netcoreapp2.1. This version is no longer supported by Microsoft (https://dotnet.microsoft.com/platform/support/policy/dotnet-core, dotnet/announcements#190).
Removing it would simplify the build. Users still targeting netcoreapp2.1 could still use the netstandard2.0 library (at the cost of falling back to an older implementation in SocketByteHandler).
It would make sense to implement this for 2.0 along with the other breaking changes.