Allow configuring batch size of UpdateAllMirrorsWorker

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A corrective action of https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7173

We observed in gitlab-com/gl-infra/production#937 (closed) that when Redis is under a lot of stress, it can take UpdateAllMirrorsWorker considerable time (we observed over 100 seconds) to enqueue all ProjectImportScheduleWorker which could lead to the piling up of overdue mirrors.

Making the batch size configurable could help us mitigate such piling up by enqueuing more jobs as the enqueuing time, we also observed, doesn't change much with the number of we enqueue.