[BUG][Java] `additionalProperties: true` generates different outputs between OAS 2.0 and 3.0

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I have an object declared as:

Application:
      type: object

      required:
        - id
        - created
      properties:
        id:
          type: integer
          description: Unique ID for this entity.
        created:
          type: string
          format: date-time
          description: The moment this entity was created.
        modified:
          type: string
          format: date-time
          description: The moment this entity was last modified.
        attributes:
          type: object
          description: Arbitrary properties associated with this entity
          additionalProperties: true

In OAS3, the generated docs and object output for this object looks like:

**attributes** | **Map<String, Object>** | Arbitrary properties associated with this entity |  [optional]

In OAS2, the generated output is:

**attributes** | **Object** | Arbitrary properties associated with this entity |  [optional]

Why do I get a different output depending on the OAS version? I'd like to keep the OAS2 output after I've migrated to OAS3, is there a config option that I missed to prevent this?

openapi-generator version

Lastest (docker)

Generation Details

cmd:

	docker run \
		--rm \
		-v "$$(pwd)/../../generated/java":/out \
		-v "$$(pwd)/../../codegen-shared/all.yaml":/tmp/src/all.yaml \
		-v "$$(pwd)/templates":/tmp/src/templates \
		-v "$$(pwd)/config.json":/tmp/src/config.json \
		-w /tmp/src \
		openapitools/openapi-generator-cli:latest generate \
		-g java \
		-i all.yaml \
		-c config.json \
		-t templates \
		-o /out \
		--artifact-version=$(artifactVersion)

config:

{
  "modelPackage": "...",
  "apiPackage": "...",
  "artifactId": "...",
  "artifactUrl": "https://github.com/...",
  "library": "okhttp-gson",
  "hideGenerationTimestamp": true,
  "gitUserId": "...",
  "gitRepoId": "..."
}

Steps to reproduce

1. Use a basic OAS3/OAS2 skeleton with an object that contains additionalProperties
2. Run the generator.

Start with the supplied OAS2/OAS3 skeleton and the Java generator invocation, comparing the generated model and documentation output for additionalProperties: true. Inspect the referenced templates and config.json, then add or run a regression case covering both specification versions; done means the output follows the agreed behavior consistently.