Use new security.authorization_checker service

Replace deprecated `security.context` with the `security.authorization_checker` service.
Use denyAccessUnlessGranted shortcut
[Security] Removed deprecated example about SecurityContext
bug #4713 [Security] Removed deprecated example about SecurityContext (iltar)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #4713).

Discussion
----------

[Security] Removed deprecated example about SecurityContext

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | ~2.6
| Fixed tickets | ~

@Einenlum noticed that there was still some documentation referring to the `SecurityContext`. This PR should remove the reference. It also fixes two occurrences where the namespace to the `AuthorizationChecker` in `versionadd` was inccorect, causing broken links to non-existing API pages.

I wasn't sure how to formulate the intro. If there are any suggestions, feel free to provide.

Commits
-------

91db61a [Security] Removed deprecated example about SecurityContext
bug #4699 Use new security.authorization_checker service (xelaris)

This PR was merged into the 2.6 branch.

Discussion
----------

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.

Commits
-------

58f4a00 Use denyAccessUnlessGranted shortcut
8ded86a Use new security.authorization_checker service
Updating one more reference of security.context that I missed in the merge
removing duplicate key
Merge branch '2.5' into 2.6

* 2.5:
  removing duplicate key