Add ops feature flag to stop pruning old events

What does this MR do and why?

Add feature flag to allow us to control if PruneOldEventsWorker does delete old events, or not.

On GitLab.com, this pruner is not doing anywhere near enough, so instead of deleting old events very slowly, we add the ability for us to stop this so that when we work on the actual solution, we have the data available.

Related issue: #24538 (closed)

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