Handle BackgroundMigration/DictionaryFile discrepancies

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138512



Merged-by: Diogo Frazão <dfrazao@gitlab.com>
Approved-by: Diogo Frazão <dfrazao@gitlab.com>
Co-authored-by: Prabakaran Murugesan <pmurugesan@gitlab.com>