Remove pipeline_execution_schedule_policy experiment flag

What does this MR do and why?

Removes the in-database pipeline_execution_schedule_policy experiment flag and replaces it with a beta GitLab feature flag (scheduled_pipeline_execution_policies) for scheduled pipeline execution policies GA rollout.

References

• #581414

Screenshots or screen recordings

Added a Beta batch:

How to set up and validate locally

1. With scheduled_pipeline_execution_policies disabled:
   • The SCHEDULE strategy option should not appear in the pipeline execution policy editor action selector.
   • RunScheduleWorker should return early without running schedules.
   • available_policy_types should exclude :pipeline_execution_schedule_policy.
2. With the FF enabled (default):
   • The SCHEDULE strategy option appears in the action selector.
   • Creating a pipeline execution schedule policy creates schedules as expected.
   • RunScheduleWorker fires pipelines for schedule policies normally.
3. Confirm no pipeline_execution_schedule_policy experiment key affects behavior in either state.

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