deck file openapi2kong broken for swagger 2.0

As of version 1.39.5 `deck file openapi2kong` fails to process swagger 2.0 specifications. This was working in 1.39.4 and prior. This is still broken in 1.40.3.

The error reported is:
```
2024/10/09 10:40:16 "msg"="error while building v3 document model \n" "error"="unable to build openapi document, supplied spec is a different version (oas2). Try 'BuildV2Model()'"
Error: failed converting OpenAPI spec 'petstore.yaml'; cannot create v3 model from document: 1 errors reported
```
(Tested using https://raw.githubusercontent.com/OAI/OpenAPI-Specification/refs/heads/main/examples/v2.0/yaml/petstore-expanded.yaml)

Reproduce the failure with `deck file openapi2kong` and the linked petstore-expanded.yaml Swagger 2.0 specification, then trace the document-model conversion that reports an OAS2 version mismatch. Done means the command successfully processes Swagger 2.0 input without the reported error and the behavior is covered by a regression check.

Written by the indexing model from the issue text.