Use new gl-button in subscriptions checkout step page

What does this MR do?

Replaces the deprecated button with the new one in subscriptions checkout page.

Screenshots (strongly suggested)

Before:	After:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #219861 (closed)