Feedback when a `ParamsKey` is deprecated

[azriel91]

The ParamsKey trait could also have a function to indicate whether each variant is deprecated or not. So that when deserializing a params file, the user can be warned about using deprecated parameters.

fn is_deprecated(self) -> bool {
    match self {
        FlowParamsKey::SomeOldParam => Err(Deprecated { hint: "Use new_param instead".to_string(), }),
        _ => Ok(()),
    }
}

This helps to inform the user to remove unnecessary params keys, and maybe migrate them.

Maybe need to think about this alongside ParamsSpecs.