Exchange /plans page with pricing

What does this MR do and why?

Replace Customers Dot plans page https://customers.gitlab.com/plans as a source of information about pricing and details about plans with https://about.gitlab.com/pricing/

Soon https://customers.gitlab.com/plans will be completely replaced and we need to update documentation before doing it

Issue: customers-gitlab-com#3645 (closed) Epic: &6806 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

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