WooCommerce Sequential Order Numbers Pro 1.21.3

* Fix - Replace deprecated `is_ajax()` usages with `wp_doing_ajax()`
* Misc - Require WooCommerce 3.9.4 or higher

* Tweak - Optimize plugin's settings layout

* Misc - Add compatibility for WooCommerce 4.7
* Misc - Require PHP 7.0 or newer