Drop support for out-of-support .NET versions

[martincostello]

The published binaries and code are starting to get bloated with adding support for .NET 8 for #2792, so we should look to remove support for target frameworks that are now completely unsupported by Microsoft.

• netcoreapp2.2
• netcoreapp3.0
• netcoreapp3.1
• net5.0
• net6.0 (from November 2024)
• net7.0 (from May 2024)

This should be done as a major version bump.

We could either drop them all at once some time after November 2024, or do it in a phased approach:

1. Drop everything not supported today (.NET < 6.0);
2. Drop .NET 6 and 7 at a future date.

[martincostello]

As part of any breaking changes made when we do this, we should also consider removing any constructor overloads we've added over time to preserve binary compatibility which also ties into #2805.