WooCommerce Store Credit 4.5.6

* Fix - Handle deprecations of WooCommerce admin navigation functions

* Fix - Address a rounding error in some configuration cases when applying multiple store credit vouchers to an order

* Tweak - Update the store credit notice styling and template
* Fix - Ensure that the notice is displayed correctly even after the store credit is removed from the cart
* Misc - Ensure the plugin does not activate if WooCommerce is not active
* Misc - Require PHP 7.4 or newer