BookingWP - WooCommerce Appointments 4.21.0

* Improvement - Add 'woocommerce_appointments_gcal_sync_order_meta' filter for order meta sync.
* Fix - Make compatibility check for navigation work correctly in older WC versions.
* Fix - Updating appointment changes from the synced gcal event can break time.
* Fix - Sync appointment changes to the calendar event's description.

* Improvement - Register menu items with WC Admin Navigation feature (deactivated by default).
* Improvement - Integration with WooCommerce PDF Invoices & Packing Slips plugin.
* Improvement - Compatibility with WooCommerce 5.1. Beta tested.
* Improvement - Compatibility with WordPress 5.7. Beta tested.
* Improvement - Updated Product Add-ons to 3.4.0.
* Improvement - Do not include addons.js when no addons are set for the product.
* Fix - Bug with gCal sync, typo.
* Fix - Pretty timestamp not calculated correctly for long duration in months/days.
* Fix - Google Calendar not updating when appointment is rescheduled.

* Improvement - Compatibility with WooCommerce 5.0.
* Improvement - Compatibility with WooCommerce Multilingual 4.11.2.
* Improvement - Updated Product Add-ons to 3.3.1.
* Improvement - Added 'woocommerce_appointment_status_changed' hook, when status transition changed.
* Improvement - Replaced empty array() instances with shorthand [] instance.
* Improvement - When billing data is empty, try getting it from shipping data.
* Improvement - Removed duplicate appointment ID queries in admin.
* Fix - PHP 8 notices for required parameters after optional ones.
* Fix - PHP 8 notices with WPML activated.
* Fix - Syncing staff and global sync for the same appointment.