Migrate to maintained OpenAPI generator

[MikeNeilson]

The original model generate plugin has been archived by the original developer.

OpenAPITools is maintaining a gradle plugin:
https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator-gradle-plugin

[MikeNeilson]

And that actually a bit of a pain in the rear to setup so for future reference:
https://github.com/opendcs/maven-central-upload/tree/main/central-api