Correct the const stabilization of <[T]>::last_chunk

`<[T]>::first_chunk` became const stable in 1.77, but `<[T]>::last_chunk` was
left out. This was fixed in 3488679, which reached stable in 1.80,
making `<[T]>::last_chunk` const stable as of that version, but it is
documented as being const stable as 1.77. While this is what should have
happened, the documentation should reflect what actually did happen.

Author: glandium

core/src/slice/mod.rs

@@ -522,7 +522,7 @@ impl<T> [T] {
     /// ```
     #[inline]
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
-    #[rustc_const_stable(feature = "slice_first_last_chunk", since = "1.77.0")]
+    #[rustc_const_stable(feature = "const_slice_last_chunk", since = "1.80.0")]
     pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]> {
         if self.len() < N {
             None