Drop support for EOL target frameworks

[abjerner]

Microsoft recommends no longer supporting .NET Standard 1.1 and .NET Standard 2.0. With .NET 9, the compiler will even trigger a warning:

https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/9.0/netstandard-warning

To follow the recommendation, and also clean up the code a bit, the package drop support for .NET Standard 1.1 and .NET Standard 2.0. This could possibly be for a v1.2.0 release.

[abjerner]

The following frameworks should probably also be dropped due to being EOL:

• .NET Framework 4.5
• .NET Framework 4.6
• .NET 5
• .NET 6
• .NET 7

So since we're currently only supporting up to .NET 7, we should then add support for .NET 8 and possibly .NET 9.

One could also argue to drop support for .NET Framework 4.7 and 4.8 since both build on top of .NET Standard 2.0, which we should still support.