[DAP-17] Rename VDAF "Preparation" to "Verification"

[jcjones]

Warning

This renames all metrics, too, so we'll need to revisit all the dashboards. I believe that is the correct move here.

• Renames the database columns, but we're assuming we're starting from clean anyway for the DAP-16 changes.
• The public AggregationJobResp.prepare_resps rename to verify_resps is a public API break for the external Rust consumers of janus_messages.

[tgeoghegan]

You'll want to throw label allow-changed-migrations on this and re-run CI (which happens on its own if you push new commits). Otherwise this is still a draft so I will wait before I review.