Updates "magic number" KPI to "growth efficiency"

Why is this change being made?

1. There is no such thing as magic.
2. Our core product is not SaaS only. We offer many installation types. The majority of our revenue currently comes from non-SaaS customers so the references to "SaaS magic numbers" seem globally inapplicable to our current business model.
3. As someone that personally invests in many different types of entities, The "magic number" is not a widely used metric for technical analysis of publicly traded positions. However, I regularly use "growth efficiency".
4. Since we aspire to be a publicly traded company at some point in the future, it feels logical that our KPIs ought to be easily understood for the audience they are targeting.

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]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in [#mr-buddies][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.