Ignore a single PHPCS rule instead of ignoring all rules

Replace the usage of `// @codingStandardsIgnoreStart` and `// @codingStandardsIgnoreEnd` with `// phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores` on WC_Gateway_Paypal_IPN_Handler::check_response(). Ignoring a single PHPCS rule is better than ignoring everything to protect the line against potential violations of the other rules.

includes/gateways/paypal/includes/class-wc-gateway-paypal-ipn-handler.php

@@ -45,9 +45,8 @@ class WC_Gateway_Paypal_IPN_Handler extends WC_Gateway_Paypal_Response {
 		if ( ! empty( $_POST ) && $this->validate_ipn() ) { // WPCS: CSRF ok.
 			$posted = wp_unslash( $_POST ); // WPCS: CSRF ok, input var ok.
 
-			// @codingStandardsIgnoreStart
+			// phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
 			do_action( 'valid-paypal-standard-ipn-request', $posted );
-			// @codingStandardsIgnoreEnd
 			exit;
 		}