Plans: Fix error from undefined longerPlanTermDefaultExperiment in Stepper use of plans step

[chriskmnds]

Related to #96174

Proposed Changes

Follow-up to #96174

We ignored usage in Stepper of the plans step from Start (/setup/onboarding/plans) and TS couldn't help flag this out (we never bothered to TS-ify plans step in Start). longerPlanTermDefaultExperiment.term would lead to uncaught type error.

Why are these changes being made?

Fix error.

Testing Instructions

• Go to /setup/onboarding/plans. Ensure it renders the plans grid.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

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[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.