Draft: Use build finished_at to add duration to the correct usage stats

What does this MR do and why?

This MR changes to use build's finished_at to add duration to the correct monthly usage stats.

Currently, the duration is being added to the month of Time.current at:

• for namespace_usage at https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/ci/minutes/update_project_and_namespace_usage_service.rb#L62
• for project_usage at https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/ci/minutes/update_project_and_namespace_usage_service.rb#L69

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Run the following code in Rails console

   build = FactoryBot.create(:ci_build, started_at: '2023-10-30T23:00:00Z', finished_at: '2023-10-30T23:59:00Z')
   project = Project.first
   namespace = project.namespace
   service = Ci::Minutes::UpdateProjectAndNamespaceUsageService.new(project.id, namespace.id, build.id)
   service.execute(888, 888 * 60)

2. Check if the duration is added to monthly usage for 2023-10-01

   project_stats = Ci::Minutes::ProjectMonthlyUsage.find_by_date('2023-10-01')
   project_stats.inspect
   
   namespace_stats = Ci::Minutes::NamespaceMonthlyUsage.find_by_date('2023-10-01')
   namespace_stats.inspect

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.

Related to #420395