Update deprecated link in documentation

The "Important notes about merge requests from forked projects" docs
had an outdated link and it was difficult to find the correct link.
This updates the docs with the correct link.

doc/ci/merge_request_pipelines/index.md

@@ -188,7 +188,7 @@ External users could steal secret variables from the parent project by modifying
 
 We're discussing a secure solution of running pipelines for merge requests
 that are submitted from forked projects,
-see [the issue about the permission extension](https://gitlab.com/gitlab-org/gitlab-foss/issues/23902).
+see [the issue about the permission extension](https://gitlab.com/gitlab-org/gitlab/-/issues/11934).
 
 ## Additional predefined variables