Feature: OpenAPI 3.1

[bbrtj]

OpenAPI 3.1 seems to be fully compatible with JSON schema. It may be possible to add a configuration field for OpenAPI version and adapt Whelk::Schema to use JSON schema validator instead of its own language. This would mean that when a new OpenAPI version is selected, all data inside Whelk schemas must be valid JSON schema, which may interfere with some system parts, since they may be coupled with Whelk::Schema internals. This must also obviously be an optional feature.