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] OAI/OpenAPI-Specification#1368
[2] https://stackoverflow.com/a/48114924/613428
[3] OAI/OpenAPI-Specification#1368 (comment)

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

Author: stephenfin