Exclude target frameworks which are EOL from test projects and add newer ones

• Add .NET 6, .NET 7, .NET 8.

• Remove .NET Core 2.0, 2.1, 3.1

• Remove .NET Frameworks 4.6.1 and 4.5.2

• Add .NET Frameworks 4.6.2, 4.7.2 and 4.8.1

This will also involve some work to make sure the automated builds support these .NET versions.