The openapi-gradle generator plugin has a strict check on the engine param to only set the parameter if handlebars is set as value. Otherwise the parameter is ignored. This is currently blocking anyone from specifiying custom template engines (https://openapi-generator.tech/docs/templating/#custom-engines) when using the official gradle plugin.

This change will set the engine param to the user-provided value, even if it's not handlebars.

The engine param was initially added in: #5686

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
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  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
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• File the PR against the correct branch: master (5.3.0), 6.0.x
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