Polylang for WooCommerce 2.1

* Requires WordPress 5.8 or higher, WC 5.6 or higher, Polylang 3.4 or higher.
* Adapt the code base to Polylang 3.4.
* Add support for XLIFF import and export with Polylang Pro.
* Add support for the product attributes lookup table.
* Bookings: Unlink person types from product instead of deleting them.
* Fix invalid order in account page if an account endpoint is set to an empty value.
* Fix checkout block redirecting to the order received page in wrong language.
* Fix shortcodes reported as missing in status report when WC pages use blocks.
* Fix legacy report for children categories.
* Fix fatal error when attempting to access the order in a function hooked to `woocommerce_email_recipient_new_order`.
* Fix: avoid a fatal error in WooCommerce Min Max Quantities.

Fix a fatal error when sending emails and the order has no language.
Fix a warning with WC Brands.
Fix image saved in wrong language in WC Brands.

* Fix import of terms with shared slugs broken by WP 6.0.
* Fix block filter by product attributes broken by WC 6.4.
* Fix fatal error when duplicating variable products and some variations have been deleted with Polylang deactivated.