Remove experiment flag ensure_pipeline_policy_pre_succeeds and finalize pipeline policy behavior

This issue tracks the removal of the experiment flag ensure_pipeline_policy_pre_succeeds that was introduced in !186968 (merged) to test the behavior where failing .pipeline-policy-pre stages fail the entire pipeline to prevent jobs with empty needs from running.

The experiment flag was implemented as part of #534302 (closed) to safely test whether failing .pipeline-policy-pre stages should fail the pipeline. After sufficient testing and evaluation, we need to make a final decision on this and remove the experiment flag.