[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.