WooCommerce Subscriptions 6.9.0

* New: Add support for importing and exporting subscription variations via the WooCommerce > Products > Export/Import tools. PR#3970
* Fix: Only show one Subscriptions settings menu item on the WC Admin navigation. PR#4054
* Fix: Prevent PHP notices on the Edit Subscription page caused accessing an undefined index. PR#4056
* Fix: Existing variations will no longer be deleted when updating a regular variable product to a subscription variable product. PR#3795
* Fix: Update the `_subtracted_base_location_tax` meta data when a subscription's line item quantity is modified. PR#4039
* Fix: Next payment date calculations when the start date and last payment date are the same. PR#4017
* Dev: Update jQuery 3.0 deprecations. PR#4015

* Fix: Pass an order status array to get_order_report_data() to prevent possible fatals by 3rd parties. PR#3930
* Fix: Change logic to allow customers to pay for failed order with limited product. PR#3947
* Fix: Prevent base length from being saved in the DB when retrieved in 'apportion_length'. PR#3954
* Fix: Prevent errors when using Product Bundles (which uses WCS_Add_Cart_Item) by introducing a WCS_Add_Cart_Item::is_switch_after_fully_reduced_prepaid_term() function. PR#3957
* Fix: Introduce an SQL Transactions helper with automatic commit/rollback on shutdown. Prevents strange behaviour when a shutdown occurs during a SQL transaction. PR#3827
* Fix: [PHP 8.0] Change maybe_retry_payment() arg name to prevent unknown variable name errors. PR#3984
* Fix: Delete '_switch_totals_calc_base_length' meta incorrectly saved in the database on upgrade. PR#3958
* Fix: Prevent WC_Order_Item_Coupon:ffsetGet errors during renewal. PR#4009
* Dev: Introduce new hooks to various Switch_Totals_Calculator functions. PR#3872

* New: Add an order note when subscriptions are deleted after the customer is deleted. PR#3783
* New: [WC Admin] Register items in new navigation menu. PR#3868
* New: [WC Admin] Add the Subscriptions setting to WC Admin's Settings navigation list. PR#3911
* Tweak: Fix capitalisation of Subscription webhook topic names so they match WooCommerce core. PR#3902
* Tweak: Refactor `wcs_help_tip()` to enable custom tooltip classes. PR#3871
* Tweak: Replace remaining uses of new WC_Order() with wc_get_order() to get an order instance. PR#3858
* Fix: Fix the amount customers are charged after switching after a previous switch which reduced the pre-paid term fully. PR#3805
* Fix: Calculate a gap payment when switching to a subscription with one length and no payment date to extend. PR#3879
* Fix: Only require cart payment if there's a subscription with a next payment date. PR#3899
* Fix: Only display the failed scheduled action warning to admin users who can view subscriptions. PR#3905
* Fix: Prefix attribute keys correctly before getting switch url. Fixes issues with auto-switch redirects removing pre-exisiting URL params. PR#3913
* Fix: Prevent reducing stock while saving a subscription on the admin edit subscription screen. PR#3926
* Fix: Use product object to read the original, unmodified length of a cart item during a switch. PR#3929
* Dev: Trigger create/update webhooks during Subscription REST API calls. PR#3919
* Dev: Add filters to enable third-parties to change the new and old payment method titles used in subscription changed payment method notes. PR#3908