fix: Gate Sidekiq dequeue logger behind env (#12790)

## Summary
- wrap the dequeue middleware registration in a boolean env flag
- document the ENABLE_SIDEKIQ_DEQUEUE_LOGGER option in .env.example

.env.example

@@ -256,6 +256,8 @@ AZURE_APP_SECRET=
 ## Change these values to fine tune performance
 # control the concurrency setting of sidekiq
 # SIDEKIQ_CONCURRENCY=10
+# Enable verbose logging each time a job is dequeued in Sidekiq
+# ENABLE_SIDEKIQ_DEQUEUE_LOGGER=false
 
 
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