Move Groups::UpdateStatistics service/worker from EE

What does this MR do and why?

This MR moves the Groups::UpdateStatisticsService and Groups::UpdateStatisticsWorker from EE to CE.

We need to do this so that they can be used to in CE models (DependencyProxy::Blobs and DependencyProxy::Manifests) to trigger the same statistics updates which will happen in !79358 (merged) as part of https://gitlab.com/gitlab-org/gitlab/-/issues/348172

Additional background

The NamespaceStatistics model and associated services/workers (being changed in this MR) were originally added into EE only as the only statistic being stored at the namespace level was an EE-level feature (group wikis). This will no longer be the case going forward as we look to expose more statistics to our customers, including non-EE stats such as DepedencyProxy size.

The NamespaceStatistics model was moved from EE -> CE in !79657 (merged)

How to set up and validate locally

1. Ensure the worker still queues successfully:

     Groups::UpdateStatisticsWorker.perform_async(some_group.id)

2. Ensure the service still works:

     Groups::UpdateStatisticsService.new(group, statistics: []).execute

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.