Nightly no longer requires error_in_core feature

    warning: the feature `error_in_core` has been stable since 1.81.0-nightly and no longer requires an attribute to enable
       --> serde/src/lib.rs:108:43
        |
    108 | #![cfg_attr(feature = "unstable", feature(error_in_core, never_type))]
        |                                           ^^^^^^^^^^^^^
        |
        = note: `#[warn(stable_features)]` on by default

serde/src/lib.rs

@@ -105,7 +105,7 @@
 // discussion of these features please refer to this issue:
 //
 //    https://github.com/serde-rs/serde/issues/812
-#![cfg_attr(feature = "unstable", feature(error_in_core, never_type))]
+#![cfg_attr(feature = "unstable", feature(never_type))]
 #![allow(unknown_lints, bare_trait_objects, deprecated)]
 // Ignored clippy and clippy_pedantic lints
 #![allow(