BookingWP - WooCommerce Appointments 4.21.0

* Improvement - Add filter 'woocommerce_appointments_gcal_sync_tay_enabled' for two way sync.
* Improvement - Add filter 'woocommerce_appointments_sync_from_gcal_event_start' for event sync.
* Improvement - Removed Select2 override classes for the admin fields.
* Improvement - WooCommerce Follow Up Emails appointment statuses trigger error.

* Improvement - Removed code for very old WooCommerce versions not supported any more.
* Fix - Payments can remain unpaid when payment gateways complete orders early.

* Improvement - Prioritize WC_Customer data over pulling customer info from orders.
* Improvement - Updated French translation.
* Improvement - Cached WC_Order and WC_Customer queries for the appointment object.
* Improvement - Better code standards (phase 1).
* Improvement - Updated Product Add-ons to 3.5.0.
* Fix - Flush cache for products with single staff.
* Fix - Don't cancel already cancelled appointment during sync.
* Fix - Recurring events can sync with 1 hour difference (DST issue).