Fix sort order on deployments list page

Release notes

Problem to solve

The deployments shown on the deployment list page sometimes don't seem to follow any sort order.

For example: https://gitlab.com/gitlab-org/gitlab/-/environments/1176344

Proposal

• We're still displaying legacy finished at. For example, the successful rows in the screenshot have NULL in finished_at column. This should have been removed. => This is done.
• Until recently, deployments that created by API didn't set finished_at. This bug doesn't exist today, but old rows would still be empty even though it's success state. We need to backfill the finished_at again. - !85952 (diffs)

Intended users

• Rachel (Release Manager)
• Devon (DevOps Engineer)
• Allison (Application Ops)

Metrics