Suggest `#[feature(min_specialization)]` instead of `#[feature(specialization)]` when user attempts to use it

[nagisa]

Right now we're suggesting something that will then immediately raise incomplete_features lint. In majority of use-cases min_specialization will likely work just as well, so we should probably suggest that as a first option first.

[nagisa]

Are we comfortable with considering this complete? From what I can tell using a feature will still suggest to add #![feature(...)] with the non-minimal feature first.