Dropping support for older versions

[Bartleby2718]

• According to https://github.com/madelson/MedallionShell/blob/master/MedallionShell/MedallionShell.csproj#L4, the TargetFrameworks for this library is netstandard1.3;netstandard2.0;net45;net46;net471.
• Per https://learn.microsoft.com/en-us/lifecycle/products/microsoft-net-framework, net461 or older versions are no longer supported.
• Per https://learn.microsoft.com/en-us/lifecycle/products/microsoft-net-and-net-core, net5.0 or older versions are no longer supported.
• Not sure about .NET Standard, byt I think we can simplify the TargetFrameworks to net462;net6.0?