Update deprecated link in documentation
What does this MR do?
The "Important notes about merge requests from forked projects" docs had an outdated link and it was difficult to find the correct link.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.
The original link was to gitlab-foss#23902 (moved), which was moved to #9713 (closed), which was closed in favor of the currently correct link: #11934 (closed).
Note: The To-Dos in #11934 (closed) have been expanded to include updating the referenced documentation.
Does this MR meet the acceptance criteria?
Conformity
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