Schedule aggregation on build artifact refresh completion

What does this MR do and why?

In an effort to make storage statistics more accurate, this MR triggers a Namespaces::ScheduleAggregationWorker when a build artifacts refresh has completed (been destroyed) so that the root level storage statistics are updated accordingly.

Refs https://gitlab.com/groups/gitlab-org/-/epics/8627 and https://gitlab.com/gitlab-org/gitlab/-/issues/372997

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