.NET Standard deprecation

[fw2568]

next major version of language.ext will require .NET 8, see also louthy/language-ext#1303

Currently we are based on .NET Standard to support both .NET framework and .NET Core.
As there is a alternative from SAP (sapnco) it should be save to assume that all users are on .NET core, but jumping directly to .NET 8 could be a hard move for some users.
Therefore .NET 8 migration should be scheduled for a version after 5, where version 5 is in between to close the gap to current language-ext features.

AB#639

[fw2568]

see this discussion for users still prefering .NET Standard: #89 (reply in thread)