[BUG] openapi meta produces code that doesn't compile

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Generator test does report that .opts() method is missing:

https://github.com/OpenAPITools/openapi-generator/blob/0204bf4ae284d936e4f7cb1ffad314c04d5df9db/modules/openapi-generator/src/main/resources/codegen/generatorClassTest.mustache#L34

DefaultGenerator doesn't seem to be extending Generator class thus is missing opts.

https://github.com/OpenAPITools/openapi-generator/blob/0204bf4ae284d936e4f7cb1ffad314c04d5df9db/modules/openapi-generator/src/main/resources/codegen/generatorClass.mustache

openapi-generator version

From source, latest release etc.

Steps to reproduce

To reproduce:

1. run openapi-generator meta.
2. mvn clean install

Related issues/PRs

Suggest a fix

Start with the generatorClassTest.mustache and generatorClass.mustache templates at the referenced commit, then run openapi-generator meta to reproduce the generated project. Inspect the generated DefaultGenerator and the expected .opts() usage. Done means the generated project compiles successfully with mvn clean install and the generator test no longer reports the missing method.