Determine how/when to drop NETFX support

[akamsteeg]

At the time of writing the most recent version of AtleX.HaveIBeenPwned still supports .NET Framework (NETFX) by targeting .NET Standard 2.0. It is not tested on version 4.6.1, which has some .NET Standard issues, or earlier, but should work with .NET 4.6.2 or up.

Unit & integration tests are run against all Long Term Support (LTS) versions At the time of writing it's .NET 4.8.1, .NET Core 3.1 & .NET 6. This guarantees a broad range of supported frameworks. An indication of that is shown on NuGet.org.

However, .NET Standard 2.0 was released in August 2017, more than five years ago. Since that time, .NET Standard was improved upon and released as .NET Standard 2.1. That version will never be implemented by .NET Framework 4.8 and since it's unlikely that there will ever be a .NET Framework 4.9(+) it's safe to assume .NET Framework will never implement .NET Standard 2.1

.NET Core 3.0 (released in 2019) and up support .NET Standard 2.1. To safely use for example a ValueTask or HashCode (see #51) we need to drop support for NETFX.

This decision is not be taken lightly. It depends also on future plans for het HaveIBeenPwned.com API.

• Are there any breaking changes planned for the API within a year of the release of VNext of this library?
• Is there no backwards compatible version of the API?
• Can we backport the changes to an earlier branch of the library?
• What's the maintenance cost of continuing support of NETFX/.NET Standard 2.0 vs. the improvements offered by more recent versions?
• Can we multi-target without siginificantly poluting the code base?

[akamsteeg]

Feedback: https://github.com/akamsteeg/AtleX.HaveIBeenPwned/issues/80#issuecomment-1445030530 The Powershell version included in Windows 11 is currently targetting NETFX.