Enum modernisation for feedback_private.idl

This CL updates the codegen attribute for the enums described in
feedback_private.idl, so it generates `enum class` types, with updated
naming conventions.

Bug: 1421546
Change-Id: If6bbfa0db66f08465db11a30e45b318048264de3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4408924


Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Claudio DeSouza <cdesouza@igalia.com>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1128847}