[BUG] [JAVA] Use Object for unsupported OneOf cases

Description

The generator generates unusable java code for complex expressions.
For example, take a oneOf that is either a string or some complex object.
The result is a an interface, which does not allow to read/write all possible values.

openapi-generator version

6.6.0

OpenAPI declaration file content or url

{
    "openapi": "3.0.1",
    "info": {
        "title": "OpenAPI definition",
        "version": "v0"
    },
    "components": {
        "schemas": {
            "Literal": {
                "type": "object",
                "oneOf": [
                    {
                        "type": "string"
                    },
                    {
                        "type": "object",
                        "properties": {
                            "@value": {
                                "type": "string"
                            },
                            "@type": {
                                "type": "string",
                                "example": "xsd:decimal"
                            },
                            "@language": {
                                "type": "string",
                                "example": "en"
                            }
                        }
                    }
                ]
            }
        }
    }
}

Suggest a fix

Shouldn't it fallback on an Object or Object Map instead of generating something incorrect?

Reproduce the supplied OpenAPI 3.0.1 declaration with the Java generator and inspect the generated model for the Literal schema. Trace how the generator handles oneOf combinations containing a string and a complex object; done means the generated Java model can represent and read/write both alternatives without producing unusable code.