Investigate Java 9+ support

[jmini]

This issue tracks effort around Java 9+ (current java version is Java 10).

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OpenAPI generator should run on Java 9+ on the classpath (modulepath could also be investigated, but It probably requires more work).

Keeping Java 8 minimal java version seems to be a good idea for the moment.

CI: Some CI Jobs should run on Java 9+.

For generated java code (jaxrs, spring, java clients, ...) we should ensure that the generated code also run on Java 9+. Generated code is probably impacted by java ee modules removal from Java SE in java 9+ (see Dependencies on JavaEE modules).

[wing328]

The project is now buildable with JDK9 via #1188

[wing328]

For JDK10, the test (project build) passed via https://app.shippable.com/github/OpenAPITools/openapi-generator/runs/3433/summary/

[wing328]

I think this can be closed.