Add `patch_id_sha` column to eventually record the `patch_id`
of the most-current `merge_request_diff` when the approval is
created. We associate to `patch_id` rather than setting up an
ActiveRecord association to a `MergeRequestDiff` since the
approval is given against a _diff_, and the same diff (from a
functional perspective) can be included in multiple `MergeRequestDiff`
records (for instance, when an MR is rebased, the diff itself doesn't
functionally change, but a new `MergeRequestDiff` record is created.)
Adding this data to `approvals` records will help us detect when an
approval is still valid, even if new `MergeRequestDiffs` are created.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/422818

Prior related work: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126920

Changelog: added