Add dropdown for AI teirs #13089074

Proposed Changes

• Show a dropdown in the AI lightbox for the user to select what tier they want
• Default to opening lightbox for AI instead of going to checkout
• Have link to support if they want a larger than the max tier

NOTES

• There is an issue with the dropdown when navigating with the keyboard, the link in the last item is not focusable. This is a deeper issue with the Dropdown component where only the list item is focusable. The logic seems complex and out of the scope of this PR to fix. I am not entirely sure if this is worth approaching the issue differently (a note below the price rather than a dropdown item) but this is what I have come up with so far
• The highlight animation around the change in request limits in the includes section will be handled separately as this PR was already large enough

Testing Instructions

1. Checkout this branch or use the Calypso Green live link and go to /pricing/
2. Click on “Get” next to the Jetpack AI product and make sure the lightbox opens up with tiers to select
   
3. Open up the dropdown and confirm all the tiers are present
   
4. Switch between them to make sure the price as well as the request limit in the includes section gets updated with each tier
   
5. Pick a tier and then select “Proceed to checkout” and make sure the correct tier is in checkout
   

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• https://wpcalypso.wordpress.com/devdocs/docs/testing/index.md 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-aUh-p2)?