Update the subscription edit product "Expire after" language so it's more clear

[james-allan]

Description

Slack: p1695716745140699-slack-C055WHLA98D

Merchants can sometimes get confused when creating a subscription product what the Subscription length/expiration is. They can interpret "Expire" to mean the subscription's renewal billing term not the full length or when the subscription will be cancelled.

This PR changes the language used to describe the subscription length/expiration fields on the edit product page to make this more clear.

Expire after → Stop renewing after Never expire → Do not stop until cancelled

^{The new Subscription product expire/length field}

How to test this PR

1. Clone this repo into your plugin directory.
2. Checkout this branch (change-expire-after-language).
3. Go to Product → Add new from your WP dashboard.
4. Select Simple Subscription from the product dropdown.
5. Notice the new label, default dropdown option and tip description.
6. Change the product type to Variable Subscription
7. In the Attributes tab create a variation.
   • Check the "Used for variations" checkbox.
8. In the variations tab create variations.
9. Verify the subscription length fields are also updated for the variations.

Product impact

• [x] Added changelog entry (or does not apply)
• [ ] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [ ] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [ ] Added deprecated functions, hooks or classes to the spreadsheet

[james-allan]

I noticed the tests were failing so I pushed up https://github.com/Automattic/woocommerce-subscriptions-core/commit/c167f1a3969002bd33dfab8c18a368a2ef456a86 to update them.

Thanks ❤️