library: Stabilize const_unsafecell_get_mut

Const-stabilizes:
- `UnsafeCell::get_mut`

library/core/src/cell.rs

@@ -2173,7 +2173,8 @@ pub const fn get(&self) -> *mut T {
     /// ```
     #[inline(always)]
     #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
-    #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
+    #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "CURRENT_RUSTC_VERSION")]
     pub const fn get_mut(&mut self) -> &mut T {
         &mut self.value
     }

library/core/src/lib.rs

@@ -158,7 +158,6 @@
 #![feature(const_typed_swap)]
 #![feature(const_ub_checks)]
 #![feature(const_unicode_case_lookup)]
-#![feature(const_unsafecell_get_mut)]
 #![feature(coverage_attribute)]
 #![feature(do_not_recommend)]
 #![feature(duration_consts_float)]