Rollup merge of #134446 - tgross35:stabilize-cell_update, r=jhpratt

Stabilize the `cell_update` feature

Included API:

```rust
impl<T: Copy> Cell<T> {
    pub fn update(&self, f: impl FnOnce(T) -> T);
}
```

FCP completed once at rust-lang/rust#50186 (comment) but the signature has since changed.

Closes: rust-lang/rust#50186

Author: matthiaskrgr

src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(rustc_private)]
 #![feature(cfg_match)]
-#![feature(cell_update)]
 #![feature(float_gamma)]
 #![feature(float_erf)]
 #![feature(map_try_insert)]