[BUG][KOTLIN-SPRING] Multipart file upload generate wrong type

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

The kotlin-spring generator generates multipart file upload attributes as org.springframework.core.io.Resource which actually cannot be handled by Spring Boot. org.springframework.web.multipart.MultipartFile should be generated instead: Uploading files guide

The spring generator handles this case correctly by applying some special case handling in the Mustache templates.

openapi-generator version

openapi-generator-maven-plugin:5.0.0

OpenAPI declaration file content or url

Multipart file upload sample spec

Steps to reproduce

1. Generate server code using the kotlin-spring generator
2. Generate server code using the spring generator
3. Note that the spring generator generates the file paramter as List<MultipartFile> while the kotlin-spring generator generates List<Resource>

Related issues/PRs

Suggest a fix

Apply the same special case handling in the templates like the spring generator

Start with the kotlin-spring Mustache templates and compare their multipart handling with the spring generator's special-case templates. Generate server code from the linked multipart sample spec using the openapi-generator Maven plugin, then verify that the file parameter is generated as List rather than List.