Enable ensure_pipeline_policy_pre_succeeds experiment by default

What does this MR do and why?

Remove the ensure_pipeline_policy_pre_succeeds experiment flag and make the .pipeline-policy-pre stage failure behavior the default.

If the .pipeline-policy-pre stage fails or all its jobs are skipped, all jobs in later stages are unconditionally skipped, regardless of scheduling type or when: configuration. This applies to all pipelines with pipeline execution policies.

References

• #558233 (comment 3129354777)

How to set up and validate locally

1. Create a project
2. Create a pipeline execution policy with a job in .pipeline-policy-pre which always fails (script: exit 1)
3. Add a project job in .gitlab-ci.yml with when: always or with needs: []
4. Ensure that the project jobs never start

MR acceptance checklist

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Related to #558233