Fix stabilization version for bindings_after_at

According to the release notes and its PR milestone, it was stabilized
in 1.56.0.

Author: camelid

Diff for: compiler/rustc_feature/src/accepted.rs

@@ -288,7 +288,7 @@ declare_features! (
     (accepted, member_constraints, "1.54.0", Some(61997), None),
     /// Allows bindings in the subpattern of a binding pattern.
     /// For example, you can write `x @ Some(y)`.
-    (accepted, bindings_after_at, "1.54.0", Some(65490), None),
+    (accepted, bindings_after_at, "1.56.0", Some(65490), None),
     /// Allows calling `transmute` in const fn
     (accepted, const_fn_transmute, "1.56.0", Some(53605), None),
     /// Allows accessing fields of unions inside `const` functions.