Use new security.authorization_checker service

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6+
| Fixed tickets |

Replace deprecated `security.context` with the `security.authorization_checker` service.
The sentence above the example says: "You can also use this directly with the ``security.authorization_checker`` service, or **via the even easier shortcut in a controller:**"

I think we should just show the helper, with a comment about the other way:

````php
$this->denyAccessUnlessGranted('edit', $post);
/* or
if (!$this->get('security.authorization_checker')->isGranted('edit', $post)) {
    throw $this->createAccessDeniedException();
} */
````

The other shortcut doesn't add much more info to this code example imo.