Backfill project stats to remove pipeline artifact

What does this MR do and why?

As described in https://gitlab.com/gitlab-org/gitlab/-/issues/412634, we need to ensure total storage sizes for ProjectStatistics and Namespaces::RootStorageStatistics are updated to remove pipeline_artifacts_size.

In this MR we achieve that by:

1. looking up ProjectStatistics that have a pipeline_artifacts_size > 0
2. re-calculating the storage_size based on the component parts that make up the value, but without pipeline_artifacts_size
3. trigger the async worker that handles the RootStorageStatistics update that will exclude the same (that class sums the related project_statistic#storage_size values, so being excluded from one means it'll be excluded from the other)

We need to have an EE override for this background migration to handle a nuance with ProjectStatistics that means we also need to exclude upload_size when calculating the new total.

This MR has used Backfill to exclude uploads_size from storage_s... (!103017 - merged) as the basis for the implementation

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