use model class for request body with additionalProperties

[martin-mfg]

fix #16294

For request bodies with additionalProperties, a generic Map is used as type instead of the proper model class. Even though the proper model class is generated. The proper model class is only used if generateAliasAsModel is true. This condition has been introduced in #4569 and is not really relevant, as far as I can tell. So I'm removing this particular condition.

I think this should go to the 8.0.x branch, but it doesn't seem to exist yet.

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[wing328]

@martin-mfg thanks for the PR. I would consider this PR as a bug fix.

Let's see if users prefer the "old behaviour" for whatever reason and we may come up with a way to fallback.

[ignacioel]

Hi @martin-mfg and @wing328,
can you have a look to the following issue #17258 ?