Auto merge of #134272 - RalfJung:destabilize-rustc_encodable_decodable, r=<try>

make rustc_encodable_decodable feature properly unstable

This has been shown in future-compat reports since Rust 1.79 (#116016). Let's see if crater still finds any issues.

Cc `@rust-lang/libs-api`

Author: bors

library/core/src/macros/mod.rs

@@ -1774,7 +1774,6 @@ pub(crate) mod builtin {
     #[unstable(
         feature = "rustc_encodable_decodable",
         issue = "none",
-        soft,
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
@@ -1788,7 +1787,6 @@ pub(crate) mod builtin {
     #[unstable(
         feature = "rustc_encodable_decodable",
         issue = "none",
-        soft,
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]

library/core/src/prelude/mod.rs

@@ -23,7 +23,6 @@ pub mod v1 {
     #[unstable(
         feature = "rustc_encodable_decodable",
         issue = "none",
-        soft,
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[allow(deprecated)]

library/std/src/prelude/mod.rs

@@ -125,7 +125,6 @@ pub mod v1 {
     #[unstable(
         feature = "rustc_encodable_decodable",
         issue = "none",
-        soft,
         reason = "derive macro for `rustc-serialize`; should not be used in new code"
     )]
     #[allow(deprecated)]