Fix status update for wrong renewal orders

[wjrosa]

Fixes 4503-gh-woocommerce/woocommerce-subscriptions

Description

Currently, if there are multiple renewal orders with the pending payment status, and you set one of them to completed, then one of the other renewal orders is changed to the processing status. This PR fixes that by checking if the order being handled is the last renewal order from the subscription. It also introduces some new helper functions for subscriptions.

How to test this PR

1. Purchase a subscription product
2. Use the "Create pending renewal order" subscription action to create 2 or more renewal orders with the pending payment status for the same subscription.
3. Set any one of the pending payment renewal orders to completed (not the last one)
4. On trunk, notice that another one of the pending payment renewal orders has been changed to processing

6. On this branch, that will not happen

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes
• Will this PR affect WooCommerce Payments? yes

[james-allan]

Hey @wjrosa. Nice PR.

The changes test well. I confirmed that processing a renewal order which isn't the latest order also causes the latest renewal order to be set to processing.

I left some inline feedback about missing function block descriptions and a suggestion about changing the wcs_get_newest_renewal_orders() to be more general use.

[james-allan]

Changes look good @wjrosa. I've just left two tiny comments. Approving as they aren't blocking. :)