Document scaling issue during Gitaly cluster migration

What does this MR do?

Document warning and solution for failed mass storage moves when using Horizontal Pod Autoscaler (HPA) with Sidekiq pods. Mass storage moves can fail silently due to pod scaling during job execution.

Problem: During mass storage moves, the following sequence can cause failures:

• Multiple storage move jobs are scheduled, creating high initial CPU load
• HPA scales up Sidekiq pods to handle the load
• As jobs progress, CPU load decreases (jobs become I/O-bound waiting on storage)
• HPA scales down pods that are still executing long-running storage move jobs
• Terminated jobs leave projects in "Temporary read-only" state without retry
• Subsequent storage moves fail silently for affected projects

Symptoms:

• Error logs show "Context deadline exceeded" without actual errors
• Projects remain stuck in read-only state
• Bulk REST API calls fail silently
• Individual project moves return read-only state errors

Related issues

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