Stabilize RFC3324 dyn upcasting coercion This PR stabilize the `trait_upcasting` feature, aka https://github.com/rust-lang/rfcs/pull/3324. The FCP was completed here: https://github.com/rust-lang/rust/issues/65991#issuecomment-1817552398. ~~And also remove the `deref_into_dyn_supertrait` lint which is now handled by dyn upcasting coercion.~~ Heavily inspired by https://github.com/rust-lang/rust/pull/101718 Fixes https://github.com/rust-lang/rust/issues/65991