[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.
and you made the same typo in the class name than the one you fixed below