#7827438 Update Boost description on pricing page lightbox

Revised the description of Boost in the Complete’s lightbox because it wasn’t conveying the product benefit.

Related to 1198218726984184-as-1204004381422738/f

Proposed Changes

Changing the text to convey product benefits

• Current text: “Automatic CSS generation.”
• New text: “Speed up your site and improve SEO with automatic Critical CSS generation.”

Testing Instructions

• Patch the changes
• Check if the text has changed and there is no Grammatical Errors
• Before:

• After:

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 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-ajp-p2)?