Clarify tier changes on the CEO pricing page

Why is this change being made?

In !60182 (merged), we modified a promise and clarified that To be clear, this promise only applies to open sourced features, features in paid tiers might move to a higher tier.

This MR proposes a corresponding change to https://about.gitlab.com/handbook/ceo/pricing and states:

• We may not "always" move features down, but we'll still default to doing so.
• The bar for moving a feature up is much higher since we're potentially reducing an existing audience for a feature rather than expanding it, which doesn't impact anyone who is currently using it.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
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