proposal: upgrade openapi definitions to 3.1

[toumorokoshi]

Today, all of aep.dev uses openapi 3.0 definitions.

Given the nascence of the project and our decision to control our own toolchains, we have some freedom to update the OAS version and not pay a painful migration in the future.

OpenAPI 3.1 has advantages, like allowing the usage of JsonSchema as a standard for resource descriptions. If we can upgrade to OAS 3.1 - we should.

The OpenAPI generator is also adding support for 3.1 as well: https://github.com/OpenAPITools/openapi-generator

[earth2marsh]

Even though 3.1 does not have strong uptake yet, I think there is a case to be made for picking 3.1, especially because it seems to me that AEPs are more about building new services in an API design-first approach (rather than upgrading legacy services).