Promote premium plan on SaaS billing page

What does this MR do and why?

Adds an experiment that redesigns the billing page for free namespaces on SaaS(user namespaces are excluded).

snowplow event dump from manual verification of tracking events added in this MR events.json

details and design are in #346273

Screenshots or screen recordings

platform	control(no change)	candidate
desktop
mobile

Candidate

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:promote_premium_billing_page)

2. Turn on billing features in the app(via rails console)

   ApplicationSetting.first.update(check_namespace_plan: true)

3. Setup CustomersDot locally
4. Configure to act as SaaS and restart gdk.
5. Create a new group.
6. Visit that group's overview page such as http://localhost:3000/flightjs
7. Find the billing link under settings.
8. Click the billing link.
9. Observe the new billing page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346273