Tracking Issue for de-stabilizing (and eventually removing?) the old serialization infrastructure (feature gate: rustc_encodable_decodable)

[RalfJung]

This tracks the progress on rust-lang/libs-team#272. That issue says that #105572 will cause rustc-serialize to stop building; it seems like a semver-compatible update was released to avoid that, so currently that crate can still be used on stable. Since Rust 1.79, the future compatibility report shows this as a warning (even for dependencies).

Implementation:

• Soft-destabilize RustcEncodable & RustcDecodable, remove from prelude in next edition #116016
• Remove rustc_encodable_decodable feature #134272

[RalfJung]

This has been removed in #134272.