fix: set correct shipping_method indexes in cart

[adtennant]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

The $index variable is never set within templates/cart/cart-shipping.php is never set and so all of the shipping methods have name=shipping_method[0] and woocommerce_after_shipping_rate always receives 0 for the index.

This change sets $index correctly within the loop:

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Go to WooCommerce > Settings > Shippings and configure a Shipping Zone with multiple shipping methods available.
2. Add at least one product to your basket.
3. Go to the Basket or Checkout page and fill in the billing address details until the shipping methods populate.
4. The shipping method input elements should have sequential name attributes of the form shipping_method[i] where i is their position in the list. You can use your browser's dev tools to verify this.

Changelog entry

Will update manually.

[github-actions]

Hi ,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[barryhughes]

Thanks! Looks good. For clarity, it may be worth noting that this change applies to the 'classic checkout' and not the default Blocks-driven checkout experience.

[barryhughes]

We'll also need to change the @version tag in the template. Given where we are in the release cycle, 9.1.0 makes sense.