[KOTLIN] add `modelMutable` additional properties parser (!11332) · Merge requests · OpenAPI Tools / openapi-generator

Created by: anantadwi13

Bugfix for #9175

It occurred because there is no modelMutable boolean parser. So the value will be treated as a string instead of a boolean.

cc @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

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```
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./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh
```
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