Change the `version` field in artifacts.json to semver

We know we're going to probably want something a little more descriptive than an incrementing integer in artifacts.json, and now is the best time to make that change rather than while we have an existing updates repo.

Here is the struct we use to parse artifacts.json:

https://github.com/oxidecomputer/omicron/blob/d8ce624cbe94e88a37bd3a5a87c422acd708afe3/nexus/src/updates.rs#L16-L26

This version field should probably be a String or Option<semver::Version>, to prevent parsing problems from breaking the entire artifact.json being deserialized (for the same reason that kind here is in an Option); the idea is that we should skip over artifacts we don't know how to reason about, because perhaps a future version of Nexus might be able to reason about them.

Here is the struct we use when Nexus tells Sled Agent to apply an update, confusingly named the same thing:

https://github.com/oxidecomputer/omicron/blob/d8ce624cbe94e88a37bd3a5a87c422acd708afe3/common/src/api/internal/nexus.rs#L80-L86

This can be more rigid since, for now, Nexus and Sled Agent are going to be built from the same omicron-common crate and agree on what the data should look like over the wire.

Start with the artifact parsing struct in nexus/src/updates.rs and the update struct in common/src/api/internal/nexus.rs, following how each version field is deserialized and sent over the wire. Decide how unknown or invalid artifact versions are skipped while preserving the stricter Nexus-to-Sled Agent representation; done means both structs use the intended semver-compatible behavior.