The `work_item_dates_sources` table/model was created to be the
definitive dates sources for any work item. The idea is to migrate all
the dates fields from issues to the `work_item_dates_sources` table
(https://gitlab.com/gitlab-org/gitlab/-/issues/461565), then remove the
dates fields from the `issues` table, making the
`work_item_dates_sources` table the Single Source of Truth (SSoT) for
work item's dates.

However, during this transition period, where we're migrating epics to
work items, we have some records with dates in the `issues` table and
others (work items epics) on the `work_item_dates_sources` table. Which
is causing some difficulties to build some filtering queries (Example:
https://gitlab.com/gitlab-org/gitlab/-/issues/457009).

To avoid these current problems, and also to avoid having to do the
large migration of all the dates data from the `issues` table to the
`work_item_dates_source` table, we decided to backfill the dates fields
from the `work_item_dates_sources` table to the issues table and ensure
that every write to the `work_item_dates_sources` table is also done on
the `issues` table, making the `issues` table the current SSoT for
dates.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/469379