Update GitLab version for duplicating dashboards

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.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.