Rename AbuseReportUpdateService to AbuseReports::ModerateUserService

What does this MR do and why?

Partially resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/167?work_item_iid=443

Rename Admin::AbuseReportUpdateService to Admin::AbuseReports::ModerateUserService so we can later use Admin::AbuseReports::UpdateService for operations that update an abuse report (e.g. update abuse report to add labels).

Also, Admin::AbuseReports::ModerateUserService is (IMHO) a more suitable name for the service because it includes banning/blocking/deleting a user in addition to optionally closing the abuse report.

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