Polylang Pro 3.6.4

* Fix media gallery messed when editing a post in the classic editor
* Fix missing script dependency on old WP versions
* Fix CSS conflict with WooCommerce Bookings for WP < 5.6
* Fix conflict resulting in '__' already defined in block-editor.js. #779
* Fix search form removed for some themes. Props Marián Kadaňka. #780
* Fix fatal error with very old versions of Yoast SEO. Props Nicola Peluchetti. #781

Add compatibility with WordPress 5.7
Remove upgrades from Polylang older than 1.8
Remove deprecated class PLL_Pointer
Pro: Hide the license keys
Pro: Fix redirect to the home page of a deactivated language
Pro: Fix synchronization of post status not working
Pro: Fix language switcher block not working in a post retrieved in REST API
Pro: Fix PO export of strings with line breaks
Pro: Fix file block title customization lost
Add a dialog box to ask a confirmation about a language change in classic and block editors
Improve browser language detection #591
Improve robustness and documentation of code
Fix media library after the language has been chnaged in the editor metabox
Fix duplicated title attribute on flag link in posts list
Fix legacy block editor language metabox compatibility with WordPress 5.6
Fix uploaded theme and plugin files in media library
Fix site title not translated in email change confirmation email
Fix remaining deprecated jQuery notices #741
Fix compatibility with GN publisher
Fix compatibility with Woodmart theme search form
Fix compatibility issue with 3rd party ajax requests since jQuery 3.3 #744
Fix CSS conflict with WooCommerce Bookings
Fix browser error when displaying an embed and using a cache plugin #757
Fix post type archive title and metadesc not translated in Yoast SEO
Fix PHP notice in REST API

* Pro: Fix translation of CPTUI plural label and description not working
* Add Spanish (Ecuador) to the list of predefined languages
* Fix typo in "WordPress" string translation group. Props Viktor Szépe #682