Remove upgrade CTAs from billing page for free personal namespaces

What does this MR do?

For .COM free personal namespaces, removes the CTAs (Call to Actions) to upgrade or start a free trial in the user preferences billing page. Those CTAs are:

• the plan cards
• the Upgrade button in the subscription table header
• the Start your free trial button

This MR implements #300345 (closed)

Screenshots

Before	After

How to test locally

You need to set up a .COM free personal namespace:

• set up a local environment that resembles that of GitLab.com -- instructions here
• create a new user for this environment via the normal sign-up flow
• login as the newly created user
• go to the user preferences billing page
• the CTAs are no more!

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• Code review guidelines
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• Style guides
• Separation of EE specific content