Ignore unknown sequences in sequence fix migration

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/160609



Merged-by: Stan Hu <stanhu@gmail.com>
Approved-by: John Mason <9717668-johnmason@users.noreply.gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Approved-by: Marius Bobin <mbobin@gitlab.com>