Stabilize const_maybe_uninit_write

Mark the following API const stable:

    impl<T> MaybeUninit<T> {
        pub const fn write(&mut self, val: T) -> &mut T;
    }

This depends on `const_mut_refs` and `const_maybe_uninit_assume_init`,
both of which have recently been stabilized.

Tracking issue: <rust-lang#63567>

Author: tgross35

library/alloc/src/lib.rs

@@ -110,7 +110,6 @@
 #![feature(const_box)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
-#![feature(const_maybe_uninit_write)]
 #![feature(const_pin)]
 #![feature(const_size_of_val)]
 #![feature(const_vec_string_slice)]

library/core/src/mem/maybe_uninit.rs

@@ -485,9 +485,10 @@ impl<T> MaybeUninit<T> {
     ///     }
     /// }
     /// ```
-    #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
-    #[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")]
     #[inline(always)]
+    #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_maybe_uninit_write", since = "CURRENT_RUSTC_VERSION")]
     pub const fn write(&mut self, val: T) -> &mut T {
         *self = MaybeUninit::new(val);
         // SAFETY: We just initialized this value.