WP Grid Builder <= 2.3.3 - Authenticated (Subscriber+) Privilege Escalation via 'key' Parameter

The WP Grid Builder plugin for WordPress is vulnerable to Privilege Escalation in all versions up to, and including, 2.3.3. This is due to missing authorization and meta key validation in the `update()` handler for the `/wp-json/wpgb/v2/metadata` REST endpoint. This makes it possible for authenticated attackers, with Subscriber-level access and above, to elevate their privileges to Administrator by updating their own `wp_capabilities` user meta with a crafted nested array payload.