Zoekt: Update Scheduling Service for replica management

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Background

We've just introduced two new columns to configure the number of replicas in #565774

Proposal

We should add a new scheduling service task to:

• Find mismatches in the target and actual number of replicas (default, and override if present)
• When such a mismatch is detected it should either add missing Search::Zoekt::Replica records or delete the extra replicas. We can either delete the ones with the higher ID or even better use states (ready takes precedence, we should keep it as much as possible)