Update CI config to support Elm 0.19

Description

Elm 0.19 has been released: https://elm-lang.org/blog/small-assets-without-the-headache

• no longer support --yes
• we will need to add elm.json or run elm init

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/877

Suggest a fix/enhancement

If anyone wants to help on this, please reply to let us know and we'll show you some good starting points.