WooCommerce Subscriptions 6.9.0

* New: Update the minimum required WooCommerce version from 3.9 to 4.4. PR#4282
* Fix: Unable to remove subscription line items via the REST API. PR#4258
* Fix: Don't show subscription related product fields when the custom variable type products are created. PR#4215
* Fix: Add consistent margins to the recurring taxes totals row on the Checkout and Cart block. PR#4273
* Fix: Fatal error due to order with no created date in order row template. PR#4273
* Fix: Fatal error on the customer payment page for renewal orders with deleted products. PR#4273
* Fix: Prevent fatal errors on the admin subscriptions screen when a subscription fails to load. PR#4290
* Fix: Incorrect message on the subscriptions table when filtering returns no results. PR#4290
* Fix: Update Cart and Checkout Block hooks to prevent deprecation warnings. PR#4280
* Tweak: Update tooltip wording when deleting product variation. PR#4273
* Tweak: Don't show an admin error notice when a store downgrades to a previous minor version of Subscriptions. PR#4273
* Tweak: Misleading order note on payment method change. PR#4273
* Dev: Moved subscription core files moved into a new subscriptions-core library (loaded from vendor/woocommerce/subscriptions-core). PR#4121
* Dev: New `WC_Subscriptions_Plugin` class to be the main Subscriptions plugin handler class. PR#4116
* Dev: Moved and deprecated 33 functions from class `WC_Subscriptions` to more suitable classes. PR#4114
* Dev: Moved switching feature related classes into its own directory (`includes/switching/`). PR#4122
* Dev: Moved payment retry feature related classes into its own directory (`includes/payment-retry/`). PR#4146
* Dev: Moved early renewal feature related classes into its own directory (`includes/early-renewals/`). PR#4148
* Dev: Moved the "Accept manual renewals" feature and settings to a new `WCS_Manual_Renewal_Manager` class. PR#4124
* Dev: Moved the "Customer Suspensions" feature and settings code to a new `WCS_Customer_Suspension_Manager` class. PR#4138
* Dev: Moved the "Drip Downloadable Content" feature and settings to a new `WCS_Drip_Downloads_Manager` class. PR#4142
* Dev: Moved the "Allow $0 initial checkout without a payment method" feature and settings to a new `WCS_Zero_Initial_Payment_Checkout_Manager` class. PR#4145
* Dev: Moved the "Limited payment recurring coupon" feature to a new `WCS_Limited_Recurring_Coupon_Manager` class. PR#4150
* Dev: Moved the custom subscription product and checkout button text feature to a new `WCS_Call_To_Action_Button_Text_Manager` class. PR#4152
* Dev: Moved the custom active and inactive subscriber roles feature to a new `WCS_Subscriber_Role_Manager` class. PR#4153
* Dev: Removed the Action Scheduler library from WC Subscriptions since it has been included in WooCommerce core since version 3.0. PR#4202

* Fix: Add back the limited subscription setting for simple subscription products (introduced in v3.1.5). PR#4214

* Fix: Update subtracted tax amounts to account for quantity changes. PR#4107
* Fix: Correctly remove limited coupons (i.e. "Active for x payments") when the coupon code is made up of only numeric characters. PR#4139
* Fix: Only set subtracted taxes on new items when the rates don't match the base location. PR#4177
* Fix: Hide variable subscription meta fields on the Edit Product page of custom variable products. PR#4193
* Fix: Use the shipping fields to get and save the edit subscription shipping field data. PR#4161
* Fix: Fix TypeError exceptions by checking for WC_Order types inside wcs_get_subscriptions_for_order(). PR#4188
* Fix: Incorrect subtracted tax calculations when updating the subscription when the store currency uses a comma decimal separator. PR#4182
* Fix: Hide the shipping address fields on checkout when the cart contains a subscription product and the 'Force shipping to billing address' setting is selected. PR#4172
* Fix: Get the signup fee for coupon calculation inclusive or excluding tax depending on store settings. PR#4166