Stabilize const_slice_flatten

Const-stabilizes `slice::as_flattened{,_mut}`:
```rust
// core::slice
impl<T, const N: usize> [[T; N]] {
    pub const fn as_flattened(&self) -> &[T];
    pub const fn as_flattened_mut(&mut self) -> &mut [T];
}
```

Tracking issue: #95629

Requires separate libs-api FCP, as per #95629 (comment).

Closes #95629.

Author: DaniPopes

library/core/src/slice/mod.rs

@@ -4784,7 +4784,7 @@ impl<T, const N: usize> [[T; N]] {
     /// assert!(empty_slice_of_arrays.as_flattened().is_empty());
     /// ```
     #[stable(feature = "slice_flatten", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_slice_flatten", issue = "95629")]
+    #[rustc_const_stable(feature = "const_slice_flatten", since = "CURRENT_RUSTC_VERSION")]
     pub const fn as_flattened(&self) -> &[T] {
         let len = if T::IS_ZST {
             self.len().checked_mul(N).expect("slice len overflow")
@@ -4821,7 +4821,7 @@ impl<T, const N: usize> [[T; N]] {
     /// assert_eq!(array, [[6, 7, 8], [9, 10, 11], [12, 13, 14]]);
     /// ```
     #[stable(feature = "slice_flatten", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_slice_flatten", issue = "95629")]
+    #[rustc_const_stable(feature = "const_slice_flatten", since = "CURRENT_RUSTC_VERSION")]
     pub const fn as_flattened_mut(&mut self) -> &mut [T] {
         let len = if T::IS_ZST {
             self.len().checked_mul(N).expect("slice len overflow")