Generated code templates are stored as resources in the .mustache format. But you can easily overwrite it by creating a modified file with the same name and adding a link to the folder if you have Maven or Gradle.
In your case, copy the api.mustache and methodBody.mustache files to your computer or project resources from https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/Java, and change them by replacing
Response<> with a
Add the item
into your Maven
<configuration> and everything should work
CLICK HERE to find out more related problems solutions.