bug: DowngradeYAML / OpenAPI 3.0.3 incorrectly produces `description` siblings for `$ref`

I'm running `vacuum` linter against specs produced by Huma and caught an issue with the default ruleset. See similar example from:
https://github.com/OAI/OpenAPI-Specification/issues/1514

```yaml
location:
description: User home address location <- error
$ref: "#/components/schemas/Location"
carLocation:
description: Where user has left his car <- error
$ref: "#/components/schemas/Location"
```

When calling `DowngradeYAML` to get an OAS 3.0.3 spec from Huma, invalid YAML like the above is produced with the `description` field set. The behavior has changed in OpenAPI 3.1.0 to allow `description` -- but it's not allowed in 3.0.3