Reinstate "Existing customer? Buy now" CTA

Step 1: What is changing in this MR?

MR reinstates the "Existing customer? Buy now" CTA on the pricing card. *

Production	Review app
https://about.gitlab.com/	WIP

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Build Variables:

• Use Contentful Preview API

Closes #3774 (closed)