[BUG][JAVA] request body parameter with explode=true need to generate separate parameters for each value of the array

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

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Example API
paths:
  /translate:
    post:
      operationId: translate
      summary: Translate text
      requestBody:
        content:
          application/x-www-form-urlencoded:
            encoding:
              text:
                explode: true
                style: form
            schema:
              type: object
              properties:
                text:
                  type: array
                  items:
                    type: string
      responses:
        '200':
          description: translations
        default:
          description: error

The code generated from the above spec should yield following request.
expected request(values form separate parameters:

curl -X 'POST' \
  'https://example.com/translate' \
  -H 'accept: */*' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -d 'text=string&text=string'

actual request (values are joined into one parameter):

curl -X 'POST' \
  'https://example.com/translate' \
  -H 'accept: */*' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -d 'text=string,string'

Please refer OAS v3 Encoding object section for more details:
https://swagger.io/specification/

openapi-generator version

5.3.1

OpenAPI declaration file content or url

See the example above.

Generation Details

Java API client generation

Steps to reproduce

Generate Java API client from the aboe spec, and send a request with that and check the request body of it.

Related issues/PRs

None

Suggest a fix

at least api.mustache template need to be fixed:
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/api.mustache#L98

Start with modules/openapi-generator/src/main/resources/Java/api.mustache, the template identified in the report, and review the OpenAPI v3 Encoding object rules for form-style exploded arrays. Generate a Java API client from the supplied specification and inspect the request body. Done means the array values are emitted as repeated text parameters such as text=string&text=string rather than one comma-joined parameter.