WooCommerce Subscriptions 6.9.0

* Fix: Prevent infinite loops while generating variable product min and max data. PR#2810
* Fix: Filter renewal order cart shipping packages, removing one time shipping products from shipping calculations. PR#2811
* Fix: Only generate persistent related order and customer subscription caches via the Transients API to avoid the possibility of retrieving out of date data stored in the option. PR#2823
* Fix: Update WC_Subscriptions_Order::maybe_autocomplete_order() to allow for it to handle orders before they have an ID (before they are inserted). PR#2784
* Fix: Adjust for possible differences in billing cycles because of rounding when calculating prorated totals for switch requests. PR#2617
* Tweak: Improve UX for variation selection when limited variable product is in cart. PR#2683
* Tweak: Remove code which supported for WC 2.5. PR#2806
* Tweak: Enforce array return value for `pre_get_users_subscriptions` filter. PR#2814

* Fix: Extend the next payment date after early renewal if there's no end date.
* Fix: Prevent throwing type hinting errors when getting related orders from cache.
* Tweak: Don't copying `_created_via` meta from subscriptions to related orders as this overrides the value set when the order was created.
* Tweak: Make `wcs_get_subscriptions_for_order()` the canonical function for retrieving subscriptions for a given order (regardless of type).

* Fix: catchable fatal error from function parameter hinting. PR#2797