Clean up schema for table merge_request_metrics

From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Wrong data type: latest_build_started_at should be timestamptz
• Wrong data type: latest_build_finished_at should be timestamptz
• Wrong data type: first_deployed_to_production_at should be timestamptz
• Wrong data type: merged_at should be timestamptz