Auto merge of #116016 - jhpratt:kill-rustc-serialize, r=ehuss

Soft-destabilize `RustcEncodable` & `RustcDecodable`, remove from prelude in next edition

cc rust-lang/libs-team#272

Any use of `RustcEncodable` and `RustcDecodable` now triggers a deny-by-default lint. The derives have been removed from the 2024 prelude. I specifically chose **not** to document this in the module-level documentation, as the presence in existing preludes is not documented (which I presume is intentional).

This does not implement the proposed change for `rustfix`, which I will be looking into shortly.

With regard to the items in the preludes being stable, this should not be an issue because #15702 has been resolved.

r? libs-api

Author: bors