Enable parallel bulk worker by default

What does this MR do and why?

This MR enables the feature flag called parallel_bulk_cron_worker by default. It has been rolled out on staging and production and showed great indexing throughput improvement.

Issue: #340963 (closed)

Rollout issue: #390361 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #390361 (closed)