Use Pajamas button in _billing_plan_actions.html.haml

What does this MR do and why?

Use Pajamas button component for button in ee/app/views/shared/billings/_billing_plan_actions.html.haml

Issue: #416052

Screenshots or screen recordings

No visual changes to the "Upgrade" button.

Before	After

How to set up and validate locally

• Have a working CDot setup locally that's link with local GDK
• In GDK, navigate to a group's billing page
• Observe the "upgrade" button in the plans section

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.