Re-named start_duo_workflows to a more generic `duo_workflow` feature flag

What does this MR do and why?

The MR re-names the narrow-scoped :start_duo_workflows flag to a more generic :duo_workflow while we're at the early stages of the Duo Workflow work

Please note that this feature flag has not been enabled in any environments so it should be fine to rename it without worrying about cleaning anything up.

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Related to #471508 (closed)