WooCommerce Account Funds 3.1.1

* Tweak - Update the database version when the plugin version changes.
* Tweak - Fixed the balance for orders paid with funds in WC 4.7.
* Tweak - Added an update script for deducting the funds from the accounts affected by the issue fixed on version 2.3.4.

* Fix - Allow changing the payment method of a subscription to "Account Funds" when this one is the unique payment gateway.

* Tweak - Tested compatibility with WC 4.8.
* Tweak - Tested compatibility with WP 5.6.
* Tweak - Don't apply the funds' discount when changing the payment method of a subscription.
* Tweak - Don't copy the account funds' metadata when creating a subscription from an order.
* Fix - The Account funds gateway was not available when changing the payment method of a subscription.
* Fix - Fixed the zero-order total displayed in recurrent orders when applying funds to new subscriptions.