Support openapi 3.1

# Context

Since February 2021, the openapi schema version 3.1.0 exists and, for the first time, is a true superset of JSON Schema. This opens the door for simpler tooling that already exists in the context of JSON Schema.

# Currently

- We run our tests against openapi documents with versions 3.0.x and 2.0.x.
- We don't know if our current dependencies (mainly openapi-backend) support 3.1.0 out of the box.

# This change will...

- [ ] Create tests and test data targeting openapi 3.1.0.
- [ ] Evaluate compatibility of dependencies with openapi 3.1.0.
- [ ] Propose a minimal change that grants openapi-cop support for openapi 3.1.0.