Update dependency_scanning_scans descriptions

What does this MR do and why?

For the metric usage_activity_by_stage.secure.dependency_scanning_scans the metric dictionary states Total number of users running Dependency Scanning Scans. However, analogous to usage_activity_by_stage_monthly.secure.dependency_scanning_scans, this metric counts security_scans, not users.

This MR aligns both metric descriptions to indicate Monthly count and Total count of dependency scanning scans.

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