Migrate to built-in popover's close button

• Integration MR for gitlab-ui!2630 (merged)

What does this MR do and why?

This migrates the trial-status-popover component to use GlPopover's built-in close button.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Run yarn install to install the development build of @gitlab/ui.
• Follow the steps at !64540 (merged).

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.