[Java] Replace "useNullForUnknownEnumValue" option with the nullable attribute

[wing328]

PR checklist

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Description of the PR

• [Java] Replace "useNullForUnknownEnumValue" option with the nullable attribute

[wing328]

Credits go to @phstudy (#1117)

[wing328]

TODO: Add tests for Java client (OAS v3 spec)

[wing328]

Upgrade Note

The option useNullForUnknownEnumValue has been removed.

Please use nullable (OAS v3) or x-nullable (OAS v2) for enum instead.