Make City and Postcode mandatory fields for Serbia

As these fields were mandatory for Serbia from the beginning, making them optional by default is causing shop owners additional effort to set them back to mandatory. This PR is making them mandatory by default.

Fixes #28366

includes/class-wc-countries.php

@@ -1230,10 +1230,10 @@ class WC_Countries {
 					),
 					'RS' => array(
 						'city'     => array(
-							'required' => false,
+							'required' => true,
 						),
 						'postcode' => array(
-							'required' => false,
+							'required' => true,
 						),
 						'state'    => array(
 							'label' => __( 'District', 'woocommerce' ),