Merge branch '521383-add-feature-flag-model-upgrade-summarize-code-review' into 'master'

Add feature flag for summarize code review model upgrade

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183213



Merged-by: Luke Duncalfe <lduncalfe@gitlab.com>
Approved-by: Michael Thomas <micthomas@gitlab.com>
Approved-by: Lucas Charles <me@lucascharles.me>
Reviewed-by: Michael Thomas <micthomas@gitlab.com>
Co-authored-by: Kinshuk Singh <kinsingh@gitlab.com>