WooCommerce Subscriptions 6.3.1

* Fix: Pass the quantity argument to woocommerce_add_cart_item_data filter. PR#3051
* Dev: Update Action Scheduler to 2.1.1. PR#3059

* Fix: Always get the subscription's customer user from post meta. Fixes issues on sites running WC 3.5 where the subscription customer was incorrectly being set to an admin user. PR#3037
* Fix: Use the current timestamp of site's timezone for comparison during synchronisation first payment date calculations. PR#3003
* Fix: Sort the user subscriptions cache to ensure subscriptions appear in the correct order on the My Account > Subscriptions page. PR#2965
* Fix: Correctly account for tax inclusive sign up fees for synced products. PR#2818
* Fix: Fix PHP 7.1.x warnings related to get_sign_up_fee() and product variations. PR#3014
* Fix: Use the correct breadcrumbs on "change payment method" and "change address" pages. PR#3008
* Fix: Fix product purchasability issues when switching between limited variable products. PR#3007
* Fix: Remove the erasure tools link from the description of the erasure setttings for users without the manage_privacy_options capability. PR#3004
* Dev: Add switch proration filters. PR#3011
* Dev: Use admin_created_subscription hook to generate download permissions. PR#3024
* Dev: Deprecated wcs_get_sites_timezone() fixes get_local_timezone() deprecated warnings. PR#3032

* Fix: Cache the retry data needs migration logic to avoid a get_posts() call on every request. PR#2994
* Fix: Load the retry store instance on WordPress init rather than on Subscriptions plugin load. Fixes errors with third-party plugins hooking into get post queries and calling undeclared WP functions. PR#3000
* Fix: Fix an undefined product ID error which could occur during a switch request when trying to access a product from the old subscription format. PR#2961
* Fix: Fix a missing closing `a` tag in the downgrade notice. PR#3002
* Dev: Add a new hook ('woocommerce_admin_created_subscription') which is triggered after a subscription is created manually via the admin screen. PR#2988
* Dev: Add new hooks to allow third-parties to customize subscription filtering behavior on the admin subscriptions table. PR#2989