Activate rustc deprecated_in_future lint by default in clippy

[Urgau]

Description

rustc has an allow-by-default lint deprecated_in_future, which does the same thing as deprecated, except that the lint is allow-by-default instead of warn or higher.

By being allow-by-default makes sense for rustc, as the intention is not to warn unnecessarily when it's not 100% required; but I think that making the lint warn-by-default with clippy would make a lot of sense, as clippy is by nature more relaxed on things that it lints on, and linting on deprecated in future usage would be something that clippy users might want.

Related to #10997 where we are trying to lint on deprecated in future items. cc @Centri3

Version

No response

Additional Labels

@rustbot label +C-enhancement

[Centri3]

I was always a bit confused why this lint was allow-by-default (or more importantly, internal); it seems it'd be something lovely, since deprecated-in-future usually doesn't come around overnight.

Also, currently the only things deprecated-in-future are the legacy constants, I thought of bringing it up there but I never got around to it.

I'm definitely in favor of this.