[BUG][kotlin][client] incomplete implementation of gson usage

[4brunu]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

The kotlin client generator, has an option to choose gson as the serialization library, but this only affects the models, the ApiClient still uses Moshi despite choosing gson.

openapi-generator version

4.20

Suggest a fix

When gson is the selected serialization library, it should be used everywhere, instead of Moshi.

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