Correct the const stabilization of last_chunk for slices

`<[T]>::last_chunk` should have become const stable as part of
<#117561>. Update the const
stability gate to reflect this.

library/core/src/slice/mod.rs

@@ -544,7 +544,7 @@ impl<T> [T] {
     /// ```
     #[inline]
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
-    #[rustc_const_unstable(feature = "const_slice_first_last_chunk", issue = "111774")]
+    #[rustc_const_stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]> {
         if self.len() < N {
             None