Review target frameworks

[thomaslevesque]

Currently the library targets net40, net45, netstandard1.3 and netstandard2.0, which means it's compatible with all currently supported versions of .NET, and also several versions that are no longer supported. All those target frameworks are also obsolete.

.NET Framework TFMs

.NET 4.0 has been out of support since forever (approximately). .NET 4.5 has also been out of support for several years.
The oldest .NET Framework version still supported by Microsoft is .NET 4.6.2.

Both .NET 4.0 and 4.5 should be dropped.
A .NET 4.6.2 TFM should be added.

.NET Standard TFMs

While not officially obsolete, .NET Standard is no longer recommended since .NET 5.0. The only .NET versions that would use the .NET Standard TFMs if a .NET 5.0 or later TFM was available are .NET Core versions, which are all out of support.

So the .NET Standard TFMs should be dropped.

.NET TFMs

.NET 6.0 is a LTS version, still supported, but support will end in November this year.
.NET 7.0 is a STS version, still supported, but support will end next month.
.NET 8.0 is a LTS version, supported until November 2026.

It would make sense to add TFMs for .NET 6.0 and 8.0. No point in bothering with STS versions.

Summary

• Remove net40, net45, netstandard1.3, netstandard2.0
• Add net462, net6.0, net8.0

This is a breaking change, so it should be done in a major release.