Update the checkout blocks hook that's attached to the process checkout flow to use the latest hook

[mattallan]

Related issue: https://github.com/woocommerce/woocommerce-subscriptions/issues/4540

Description

There is an issue where processing a Switch request on a block checkout results in a new subscription being created instead of the existing subscription being updated. This issue was caused by our WC_Subscriptions_Checkout class using a deprecated checkout block hook while our WC_Subscriptions_Switcher is using the latest store API hook.

This was causing problems because WooCommerce calls the deprecated hooks first before the latest hook resulting in WC_Subscriptions_Checkout::process_checkout running before WC_Subscriptions_Switcher::process_checkout which was creating the extra subscription correctly.

It's important that the switcher class processes the checkout first so that it can remove any items from the recurring cart to prevent subscriptions from being created on the switch.

Note

This PR removes the version_compare on \Automattic\WooCommerce\Blocks\Package::get_version() because Subscriptions supports WooCommerce 7.7 at a minimum which already comes bundled with Checkout Blocks v10.0.2.
Therefore I've removed all of the deprecated hooks and conditions checking for older versions of checkout blocks that we no longer support.

I'm not sure if there are any concerns with stores installing an older version of checkout blocks as a standalone plugin but I don't think we should cater to that.
Open to discuss this :)

How to test this PR

Important

To follow these testing instructions you'll need to have the main Woo Subscriptions plugin active to access switching.
There's also another bug that prevents you from submitting a $0 switch checkout when using the block checkout. This will be addressed in a PR to the main repository.

1. Make sure you have a block checkout page available to test
2. Enable switching between variations in the WC > Settings > Subscriptions
3. Purchase a variable subscription product
4. Go to My Account > My Subscription
5. Click on the Upgrade or Downgrade button
6. Select a different variation and complete the switch checkout
7. On trunk you'll notice you now have two active subscriptions for the same variable product
   
8. On this branch, you will only have 1 subscription.
   In order to be able to bipass the checkout erroring with "No valid payment method" I had to stub WC_Subscriptions_Order::order_needs_payment() to return false on the first line.

Product impact

• 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]

Testing in conjunction with https://github.com/woocommerce/woocommerce-subscriptions/pull/4567.

The new hooks used in this PR were added to WooCommerce core in v6.4 (woocommerce/woocommerce#32075) and given we only support versions of WC back to 7.7, this update is safe.