REST API: Fix incorrect capability check on term create.

Change the capability check used in `WP_REST_Terms_Controller` when creating a new term is attempted, from `manage_terms` to `edit_terms`. This matches the behavior within the WordPress admin. See #35614.

Props johnbillion, rmccue, rachelbaker, helen, jorbin, SergeyBiryukov.

Fixes #38958.
Built from https://develop.svn.wordpress.org/trunk@39402


git-svn-id: https://core.svn.wordpress.org/trunk@39342 1a063a9b-81f0-0310-95a4-ce76da25c4cd