fix: upgrade openapi-generator-cli version to 6.6.0 for elixir

[tobbbles]

Fixes #282.

Related Issue or Design Document

Checklist

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• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
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Further comments

There are still some annoying bugs and inconsistencies in the Elixir implementation of openapi-generator. My fingers are crossed for version 7.0.0 and the following PR OpenAPITools/openapi-generator#15746, which will introduce some breaking changes to any users currently using the Elixir clients, but brings massive improvements and overhauls.

[aeneasr]

Awesome, thank you!

By the way, there's no need really to update the files in clients as those are autogenerated anyways :)