fix oapi3'ism in identity service

[saces]

range definition for response codes is an oapi3 feature.
it goes without warning with this patch,
may the experts decide about the numbers to use ;)

[anoadragon453]

The context is that having 3xx instead of 300 can break some tools processing our spec.

I believe we use 400 to mean 4xx in other places of the spec, so this doesn't seem entirely unreasonable a change.

[turt2live]

We use the 2xx format elsewhere in the spec. I'm tempted to just call it part of our "extensions" for when we switch to OpenAPI 3.

[KitsuneRal]

I'd also just dub it one more extension and consider it one more small nudge towards OpenAPI 3 migration.

[turt2live]

I've opened the documentation as #2561