clarify status of duplicate issues

Problem to solve

The GitLab feature list explicitly states that duplicate issues are part of free and core which I assume means they are part of the free version of GitLab (both speech and beer). Yet if you click on the very documentation link right below that feature, it brings you to the quick actions documentation which has a warning saying the feature is only available in "Starter" and "Bronze".

So which one is it?

Proposal

Make the docs correctly state the license needed to mark issues as duplicate.

Who can address the issue

No idea. :)

Other links/references

I haven't traced exactly the history of those docs, as it's hard for me to figure out which source code repository is associated with a page...