WooCommerce USPS Shipping Method 5.2.0

* Tweak - WooCommerce 9.9 compatibility.

* Replace ubuntu-20.04 with ubuntu-latest in GitHub Actions.
* Update Requires headers for WooCommerce compatibility.

* Tweak - WooCommerce 9.7 Compatibility.

* Tweak - PHP 8.4 Compatibility.

* Tweak - WordPress 6.7 and WooCommerce 9.3 compatibility.

* Tweak - WordPress 6.6 and WooCommerce 9.0 Compatibility.

* Fix - Corrected deprecated null parameter to comply with updated PHP standards.

Fix - Removed WC_Boxpack class_exists check as it is no longer needed.

Fix - Apply WordPress coding standards.

Add - Prefix Box-Packer namespacing.