add support for OAS 3.2.0

[mattpolzin]

This change involves combing the release notes and updating where new features have been added.

I am not sure (yet) if this can be done in a non-breaking way (for a 4.x release).

I think the ideal way to do this will be to create additive changes that aren't breaking if possible and then throw in a validation that will fail if any features added with OAS 3.2 are used with OAS 3.1 or 3.0 being specified as the document version.

If all else can be done in a non-breaking way, then the new version can also be specified in a non-breaking way for OpenAPIKit 4.x. For OpenAPIKit 5.x the new version can be moved into the base enum (breaking).

[mattpolzin]

This is going to be breaking (v.5.0) and I am going to reduce scope of this particular issue to just include (a) allowing the new version to be used and (b) supporting an initial small subset of the changes made in OAS 3.2.0. I will introduce more of the changes then in follow-on issues and PRs.