Unpin openapi-spec-validator

We have this pinned in `ci/pip_requirements.txt` to avoid installing `openapi-schema-validator`, which has an unexpected issue with `required`, and results in changes of valid datetime representations. Ideally, we *do not* pin `openapi-spec-validator` and adjust our schema to conform with the latest versions of that package (which seems to be in flux as of recent).

Start with ci/pip_requirements.txt and inspect the OpenAPI schema fields involving required values and datetime representations. Check the schema against the latest openapi-spec-validator behavior; done means the dependency is unpinned, validation succeeds, and valid datetime representations are unchanged.

Written by the indexing model from the issue text.