DB schema: Remove `update` variant from `database_kind`

DatasetKind::Update and its associated database enum variant update were added when we thought TUF repo artifacts would be stored on U.2 drives (and therefore need datasets set up by reconfigurator). We've since changed to storing them on the M.2s, and sled-agent sets up the necessary datasets autonomously.

In an #8301 we're removing the DatasetKind::Update Rust variant. However, removing variants from a database variant is an annoying amount of work, so we've punted on this for now in favor of urgency. The practical impact of this should be small: if we read an update value from the database (which should never happen), we'll return a runtime error. If we don't get to this on its own merits, the next time we go through the process of modifying dataset_kind for some other reason, we should prune this variant.

Start with the schema/crdb guide on changing enum variants and compare the database change with #8301, which removes DatasetKind::Update. Trace the database_kind enum and its conversion paths. Done means the update variant is pruned through the required schema process and unexpected update values produce the stated runtime error.