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.
I agree to use `$this->isGranted()` or `denyAccessUnlessGranted` is better in practice. I think the example is more about demonstrating the usage of the `security.authorization_checker` service as opposite to the `@Security` annotation. IMO the shortcut would hide that essence. I am wavering, a thin and readable controller on one side, an obvious example on the other side.
What about adding a hint to the shortcuts only?