Use openapi generate from node

[dpash]

I had some issues getting the project to build, with finding the correct version of openapi generator.

To make it slightly easier to set up the environment, this commit uses @openapitools/openapi-generator-cli to fetch the relevant version. We're stopping at 6.6.0 as later versions have issues with the project. We also need Java 15 or earlier, as Java 16 closes access to a number of classes that OpenAPI uses (via the Java Moustache library).