Add update action and update service for Abuse Reports

What does this MR do and why?

Add Admin::AbuseReportsController#update action and Admin::AbuseReportUpdateService for Abuse Reports.

This provides support for the Abuse Reports detail view actions drawer.

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/338+

Epic: https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/8+

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