Update GitLab version for duplicating dashboards

Merged Marcin Sedlak-Jakubowski (OOO | back on 4 July) requested to merge patch-73 into master

What does this MR do?

Fix the version number in which this was introduced.

Docs: https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#duplicating-a-gitlab-defined-dashboard
12.8 source (discussion): gitlab-com/www-gitlab-com!39892 (comment 287121777)

Related issues

Related to #37238 (closed)

Documentation is pointing to this issue. If this feature was added in 12.8, then the issue's milestone should be changed to 12.7 too.

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Edited by Marcia Ramos