WooCommerce Sequential Order Numbers Pro 1.21.2

* Fix - Return correct order numbers in WooCommerce REST API responses for v3 and v2
* Misc - Drop support for PHP v5.2: PHP v5.3 is now the minimum supported version
* Misc - Update SkyVerge plugin framework to version 5.3
* Misc - Change the plugin main file to woocommerce-sequential-order-numbers-pro.php to match container directory

* Fix - Show leading zeros when {N} and {S} placeholders are used in sample order number preview
* Misc - Add support for WooCommerce 3.5

* Misc - Add support for WooCommerce 3.4