Use `wp_is_serving_rest_request` once we drop support for WP 6.4

[leonidasmi]

WP 6.5 introduced the wp_is_serving_rest_request() function (and the wp_is_rest_endpoint() function) that can rcompletely replace our Request_Helper class, so let's do that and use the core wp_is_serving_rest_request() function instead.

Things to take note of:

• check references of the Request_Helper class and the REST_REQUEST define, in all our plugins, so we replace all of them.
• check if in some cases, wp_is_rest_endpoint() is the more appropriate function to use