fix(modal-checkout): use post checkout text option to populate thank you page text

[ ] Have you followed the Newspack Contributing guideline?
[ ] Does your code follow the WordPress' coding standards and VIP Go coding standards?
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This PR uses the new post checkout success text setting added in the main plugin PR linked above.

Screenshot 2024-11-07 at 17 09 04

As admin, ensure the Prompt logged out readers to sign in or register a new account before checkout setting is DISABLED via Newspack > Engagement > Show Advanced Settings > Checkout Configuration
As a logged out reader, go through checkout using a new email address not associated with an existing user account until you get to the thankyou modal.
Verify whatever text in the post-checkout registration success message setting appears in the thankyou modal
As a logged in reader, go through checkout again until you get to the thankyou modal.
Verify whatever text in the post-checkout success message setting appears in the thankyou modal
As a logged out reader, go through checkout again, this time using an email address associated with an existing user, until you get to the thankyou modal.
Verify whatever text in the post-checkout success message setting appears in the thankyou modal
As admin, enable the toggle from step 1
Go through checkout as a logged out new user, a logged out existing user, and a logged in reader
In all cases confirm the post-checkout success message appears in the thankyou modal

[ ] Have you added an explanation of what your changes do and why you'd like us to include them?
[ ] Have you written new tests for your changes, as applicable?
[ ] Have you successfully ran tests with your changes locally?

Automattic / newspack-blocks