Drop support for .NET framework 3.5

[milkshakeuk]

Is your feature request related to a problem? Please describe.
.NET framework 3.5 is considered legacy and there is a development overhead to its continued support.

Describe the solution you'd like
Drop support for .NET framework 3.5 with netstandard2.0 being the minimum supported target. This covers projects targeting .NET framework 4.6.1+ and .NET Core 2.0+ up to and including .NET 6

Describe alternatives you've considered
Continued support for .NET framework 3.5

Additional context
Support for .NET framework 3.5 shouldn't really be required in 2021 as hopefully most new and existing projects have moved on from .NET framework 3.5 and if not, they can use any version of nHapi up to v3.X.X or maybe take an opportunity to migrate to a later supported version of .NET.