[BUG][JAVA][Spring] Form passing with application/x-www-form-urlencoded uses @RequestPart instead of @RequestParam

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?
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Description

Using application/x-www-form-urlencoded as the requestbody generates spring methods with @RequestParts instead of @RequestParam

More info:
https://github.com/swagger-api/swagger-codegen/issues/6438

openapi-generator version

5.0.0-beta2

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: 'app'
  version: 0.0.1
servers:
  - url: http://localhost:8080/api
    description: Development server
paths:
  /authorize:
    post:
      summary: Authorization Endpoint
      operationId: authorize
      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                grant_type:
                  type: string

Generation Details

I'm generating the server code with openapi-generator-maven-plugin .

Suggest a fix

Use @RequestParam for application/x-www-form-urlencoded instead of @RequestPart since it's not a file

Start by reproducing the generated Spring server with the supplied OpenAPI declaration and the openapi-generator-maven-plugin. Trace how the application/x-www-form-urlencoded request body is mapped to method parameters. Done means generated form fields use @RequestParam rather than @RequestPart, without changing file-upload handling.