Stabilize the cell_update feature

FCP complete at [1].

Link: rust-lang#50186 (comment) [1]
Closes: rust-lang#50186

Author: tgross35

library/core/src/cell.rs

@@ -549,8 +549,6 @@ impl<T: Copy> Cell<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(cell_update)]
-    ///
     /// use std::cell::Cell;
     ///
     /// let c = Cell::new(5);
@@ -560,7 +558,7 @@ impl<T: Copy> Cell<T> {
     /// assert_eq!(c.get(), 6);
     /// ```
     #[inline]
-    #[unstable(feature = "cell_update", issue = "50186")]
+    #[stable(feature = "cell_update", since = "CURRENT_RUSTC_VERSION")]
     pub fn update<F>(&self, f: F) -> T
     where
         F: FnOnce(T) -> T,

library/core/tests/lib.rs

@@ -11,7 +11,6 @@
 #![feature(async_iter_from_iter)]
 #![feature(async_iterator)]
 #![feature(bigint_helper_methods)]
-#![feature(cell_update)]
 #![feature(clone_to_uninit)]
 #![feature(const_black_box)]
 #![feature(const_eval_select)]

src/tools/miri/src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(rustc_private)]
-#![feature(cell_update)]
 #![feature(float_gamma)]
 #![feature(map_try_insert)]
 #![feature(never_type)]