On stable, suggest removing `#![feature]` for features that have been stabilized

I don't know how to test this. I confirmed locally that this gives the
appropriate help with `channel = "beta"`:

```
error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:2:1
  |
2 | #![feature(min_const_generics)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the attribute
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:3:1
  |
3 | #![feature(min_const_generics, min_specialization)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: the feature `min_const_generics` has been stable since 1.51.0 and no longer requires an attribute to enable

error[E0554]: `#![feature]` may not be used on the beta release channel
 --> src/lib.rs:4:1
  |
4 | #![feature(box_patterns)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
```

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