Duo_workflow rake specs parent_ts dependency by using new uuid_v7 from gitlab utils

The following discussion from !203701 (merged) should be addressed:

• @GitLabDuo started a discussion: (+2 comments)

  The parent_ts field is being set to a UUID v7 when step > 0, but this appears to be a logical error. The parent_ts field should reference the thread_ts of a parent checkpoint, not generate a new random UUID. This will create orphaned references that don't point to actual parent checkpoints.

            parent_ts: step > 0 ? previous_checkpoint_thread_ts : nil,