Merge branch '434580-remove-elastic_index_use_trigger_indexing-ff' into 'master'

Remove elastic_index_use_trigger_indexing FF

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159435



Merged-by: Terri Chu <tchu@gitlab.com>
Approved-by: mo khan <mo@mokhan.ca>
Approved-by: Rutger Wessels <rwessels@gitlab.com>
Reviewed-by: mo khan <mo@mokhan.ca>