Backfill Issues[start_date,due_date] with WorkItems::DatesSource

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157993

Merged-by: Adam Hegyi <ahegyi@gitlab.com>
Approved-by: Felipe Artur <fcardozo@gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Reviewed-by: Felipe Artur <fcardozo@gitlab.com>
Reviewed-by: Kassio Borges <kborges@gitlab.com>
Reviewed-by: Alexandru Croitor <acroitor@gitlab.com>
Reviewed-by: charlie ablett <cablett@gitlab.com>
Co-authored-by: Kassio Borges <kassioborgesm@gmail.com>
Co-authored-by: Kassio Borges <kborges@gitlab.com>