BookingWP - WooCommerce Appointments 4.21.0

* Improvement - Compatibility with WordPress 5.9.
* Improvement - Updated compatibility with Polylang for WooCommerce 1.6.2.
* Improvement - Updated Product Add-ons to 4.5.0.
* Fix - Polylang staff costs and quantity erased on translated products.
* Fix - Datepicker in admin shows html instead of icon from WP 5.9.
* Fix - WC product Addons translation not working.

* Improvement - Added 'woocommerce_appointments_product_defaults' to filter product defaults.
* Improvement - Added better compatibility with the Square payment plugin.
* Improvement - Compatibility with WooCommerce 6.1.
* Improvement - Updated Product Add-ons to 4.4.0.
* Fix - Pagination at My Appointments account section.

* Improvement - Replaced more array() instances with shorthand [] instance.
* Improvement - Updated Spanish translations.
* Improvement - Added new translated strings to all languages.
* Improvement - WooCommerce Payments multicurrency support for add-on fields.
* Fix - Some strings in "appointment-summary-list.php" are not translatable since 4.14.1.
* Fix - Admin calendar should include the tax amount for listed appointments.
* Fix - All new products (even simple) have the 'Virtual' option checked.
* Fix - Saved customer ID for the appointment not shown.
* Fix - Multi-day duration does not highlight selected days on disabled dates.
* Fix - Pagination at My Appointments account section.