@keystone-6/core vulnerable to field-level access-control bypass for multiselect field

@keystone-6/core is a core package for Keystone 6, a content management system for Node.js. Starting with version 2.2.0 and prior to version 2.3.1, users who expected their `multiselect` fields to use the field-level access control - if configured - are vulnerable to their field-level access control not being used. List-level access control is not affected. Field-level access control for fields other than `multiselect` are not affected. Version 2.3.1 contains a fix for this issue. As a workaround, stop using the `multiselect` field.