Merge branch '521400-add-feature-flag-model-upgrade-summarize-merge-request' into 'master'

Add feature flag for summarize merge request model upgrade

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182958



Merged-by: Gary Holtz <gholtz@gitlab.com>
Approved-by: Michael Thomas <micthomas@gitlab.com>
Approved-by: Gary Holtz <gholtz@gitlab.com>
Co-authored-by: Kinshuk Singh <kinsingh@gitlab.com>