BUG: redundant "last updated" timestamp in forecast and RT snapshots

[catkins-miso]

I think this probably is a result of DRYing up the source/snapshot-provenance. Changing this is technically a spec change, but I hope we can treat it as just a clean-up.

Relatedly, the forecast examples have a superfluous "updated-time"
https://github.com/trolie/spec/blame/409e215cd7dddfae2efcd5f344faadfd826c8f53/docs/example-narratives/examples/forecast-limits-detailed.json#L48

This should be removed.

[catkins-miso]

FWIW, the superfluous property wasn't caught by the linter because we couldn't use additionalProperties: false on the rating schema , as discussed here https://stackoverflow.com/a/23001194

Moving to OpenAPI 3.1.0 would allow us to use unevaluatedProperties: false which has the desired effect.

Another option would be to use <<: *anchor as we've done in other places rather than the built-in allOf method.