Remove enable_dependency_bump_breaking_changes flag

What does this MR do and why?

Removes the enable_dependency_bump_breaking_changes flag (beta, default_enabled: true, 19.2). Rollout complete.

Feature stays, still beta, gated only by:

• project setting duo_dependency_bump_breaking_changes_enabled
• Ultimate licence (ai_features)
• foundational flow availability

Out of scope: enable_dependency_bump_breaking_changes_pipeline_tracking untouched (own MR). dependency_bump_web_search stays documented.

Docs: 19.4 history entry added, prerequisite removed.

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References

• Feature flag rollout issue: #604558 (closed)
• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/603392
• Flag introduced by: !241435 (merged)