Enable scheduled and duo automations feature flags by default

What does this MR do and why?

Enables scheduled automations and the redesigned Automations list by default in 19.6 by moving three feature flags to the beta type with default_enabled: true.

Flag	From	To
ai_flow_schedules	gitlab_com_derisk	beta
autonomous_service_account_execution	gitlab_com_derisk	beta
duo_automations	wip (EE)	beta (EE)

Note: This does not make the feature available with no customer action

Scheduled automations [stay gated](https://gitlab.com/gitlab org/gitlab/-/blob/master/ee/app/models/ai/flow_trigger.rb#L540) on the root namespace's experiment and beta features setting.

This other MR: !270212 (merged) depends on this one getting merged.

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