Convert to OpenAPI 3.1

Convert to OpenAPI 3.1

OpenAPI 3.0 has some unspecified behavior regarding the combination of
reference objects with 'nullable' [1]. Despite what random StackOverflow
answers [2] suggest, combining nullable with '$ref' still doesn't work.
Do what's suggested in the issue reporting this behavior [3] and upgrade
to OpenAPI 3.1, allowing us to work around this.

[1] https://github.com/OAI/OpenAPI-Specification/issues/1368
[2] https://stackoverflow.com/a/48114924/613428
[3] https://github.com/OAI/OpenAPI-Specification/issues/1368#issuecomment-930112844

Signed-off-by: Stephen Finucane <stephen@that.guru>