Update Custom WooCommerce Payment Gateway Plugin for Nexi Netaxe

I have an outdated custom WordPress plugin that worked as a payment gateway in WooCommerce, integrating with Nexi Netaxept. The plugin is no longer compatible with the latest versions of WordPress and WooCommerce.

**Goal**
Update and fix the existing plugin so that it:

1) Is fully compatible with the latest stable versions of WordPress and WooCommerce
2) Works correctly with Nexi Netaxept as a payment gateway
3) Follows current WordPress/WooCommerce coding standards and best practices

**Available Resources**
1) The existing plugin (attached)
2) Token/credentials for Nexi’s test environment

**Requirements**
1) Clean, well-documented code