Arity for Option type can't be set with .NET Framework targets

[ralish]

The Arity property of the Option type used to have a standard setter but it was changed to an init only setter a while back. Init only setters were introduced in C# 9.0, however, .NET Framework targets are limited to C# 7.3. This results in it being impossible to set the Arity of an Option as the only mechanism isn't supported by the C# language version. Apologies if I'm missing something obvious.