* Misc-Add PHP 8 support

* Fix -Replace deprecated jquery.bind() and jquery.parseJSON() functions.

* Tweak: Improve the missing customer error message displayed for third-party built checkouts when registration is disabled on checkout. PR#3893
* Fix: Remove the possibility of a negative calculation of prepaid days on subscription downgrades. PR#3881
* Fix: Fix issues preventing stores using the $0 Initial Checkout feature from being able to checkout. PR#3887
* Fix: Remove potential fatal error when processing end-of-prepaid term scheduled actions caused by the subscription not existing. PR#3875
* Fix: Trigger payment complete actions after renewing early via modal. PR#3888
* Fix: Performance improvements to limited products running slow queries on the shop page. PR#3895
* Fix: [PHP 8.0] Only pass array values to call_user_func_array calls. PR#3884
* Fix: [PHP 8.0] Replaces certain uses of method_exists() with is_callable(). PR#3892

* Fix-PHP 8 compatibility.

* Misc-Add PHP 8 support

* Tweak-WordPress 5.6 compatibility.
* Tweak-WC 4.7 compatibility.