Upgrade Microsoft.AspNetCore.OpenAPI and add support for OpenAPI v3.1

[captainsafia]

The Microsoft.OpenApi team is gearing up to ship a new major version of the implementation that supports serializing OpenAPI documents to v3.1.

This issue tracks updating the OpenAPI implementation in Microsoft.AspNetCore.OpenApi to consume the new version of this dependency and making the necessary changes to target their new OpenApiSchema definition.

The new OpenAPI schema definition contains better built-ins for recursive references that we can consider depending on instead of our current $ref based approach. The new approach also exposes JSON Schema's underlying $defs structure for denoting common schemas that we can consider using in lieu of the current mapping structure.