Change approved to merged in Contribution Analytics
Previously Contribution Analytics listed "approved" merge requests, which meant merged. That use of non-standard terminology was confusing to users, so this changes it to "merged" for clarity.
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![doc/user/group/contribution_analytics/img/group_stats_table.png](https://lab.node.vifib.com/nexedi/gitlab-ce/-/raw/7745bf3ace666c11cc41cec95de04c4ee93b4e69/doc/user/group/contribution_analytics/img/group_stats_table.png)