Remove ensure_pipeline_policy_pre_succeeds feature flag

What does this MR do and why?

Removes the ensure_pipeline_policy_pre_succeeds feature flag, which was introduced in GitLab 18.10 and enabled by default. The behavior it controlled — skipping all later pipeline jobs when the .pipeline-policy-pre stage fails — is now the unconditional default as of GitLab 19.0.

Changes:

• Deletes config/feature_flags/beta/ensure_pipeline_policy_pre_succeeds.yml
• Removes the Feature.enabled? guard in ReservedStageStatusCalculationService, making calculate_reserved_pre_stage_status always run (removing the calculate_dag_only_status fallback path)
• Updates documentation to remove the Beta status callout, the [!flag] admonition, and the note about disabling the flag; adds a history entry noting the flag was removed in 19.0
• Removes all stub_feature_flags(ensure_pipeline_policy_pre_succeeds: false) test contexts from the spec files

References

• !225559 (merged) (original flag introduction)
• #558233 (closed)

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