WooCommerce Store Credit 4.5.6

* Feature - Allow customers to choose the amount of credit to purchase.
* Tweak - Display the credit amount when a Store Credit product is added to the cart.
* Tweak - Updated the 'Store Credit' description text on the 'My Account' page.
* Tweak - Tested compatibility up to WC 6.3.
* Tweak - Tested compatibility with WP 5.9.
* Dev - Use a WC background process to handle the plugin updates.
* Dev - Use a trait instead of a class for implementing the Singleton pattern.
* Dev - Dropped deprecated plugin code up to version 3.9.
* Dev - Removed deprecated constants `WC_STORE_CREDIT_PLUS_VERSION`, `WC_STORE_CREDIT_PLUGIN_DIR`, and `WC_STORE_CREDIT_PLUGIN_URL`.
* Dev - Updated the minimum requirements to WC 3.7, WP 4.9, and PHP 5.6

* Feature - Added setting to edit the title to Send to someone of Store Credit products.
* Tweak - Tested compatibility with WC 6.0.

* Tweak - Tested compatibility with WC 5.9.
* Tweak - Improved the distribution of the coupon credit over the cart items.
* Fix - Fixed wrong discount when the cart contains many low-priced items.