[Bug][Java][Spring] Dynamic query parameters with multiple values generates wrong type

Description

With additionalProperties and explode it's possible to allow dynamic query parameters with multiple values. The spring generator creates a Map<String, List<String>> for it. Spring supports such query parameters since Spring 3.x but it needs a MultiValueMap.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: BugExample.yml
  version: '1.0'
servers:
  - url: 'http://localhost:3000'
paths:
  '/users':
    parameters:
      - name: filters
        in: query
        required: true
        schema:
          type: object
          additionalProperties:
            type: array
            items:
              type: string
          example:
            firstName: 
              - Alice
            lastName:
              - Smith
              - Jones
        style: form
        explode: true
    get:
      summary: Get all users matching the filter
      responses:
        '200':
          description: Users Found
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
              examples:
                Get Users:
                  value:
                    - id: 142
                      firstName: Alice
                      lastName: Smith
                      email: alice.smith@gmail.com
                      dateOfBirth: '1997-10-31'
                      emailVerified: true
                      signUpDate: '2019-08-24'
                    - id: 123
                      firstName: Alice
                      lastName: Jones
                      email: alice.jones@gmail.com
                      dateOfBirth: '1997-10-30'
                      emailVerified: true
                      signUpDate: '2019-08-25'
      operationId: get-users-filtered
      description: Retrieve all users matching the filters.
components:
  schemas:
    User:
      title: User
      type: object
      description: ''
      examples:
        - id: 142
          firstName: Alice
          lastName: Smith
          email: alice.smith@gmail.com
          dateOfBirth: '1997-10-31'
          emailVerified: true
          signUpDate: '2019-08-24'
      properties:
        id:
          type: integer
          description: Unique identifier for the given user.
        firstName:
          type: string
        lastName:
          type: string
        email:
          type: string
          format: email
        dateOfBirth:
          type: string
          format: date
          example: '1997-10-31'
        emailVerified:
          type: boolean
          description: Set to true if the user's email has been verified.
        createDate:
          type: string
          format: date
          description: The date that the user was created.
      required:
        - id
        - firstName
        - lastName
        - email
        - emailVerified

Command line used for generation

OpenAPI Generator maven plugin:

<plugin>
				<groupId>org.openapitools</groupId>
				<artifactId>openapi-generator-maven-plugin</artifactId>
				<version>6.0.1</version>
				<executions>
					<!-- Fake Source API -->
					<execution>
						<id>fake-source-service</id>
						<goals>
							<goal>generate</goal>
						</goals>
						<configuration>
							<!-- specify the openapi yaml -->
							<inputSpec>${project.basedir}/src/main/resources/openapi/api/example.yml
							</inputSpec>

							<!-- target to generate java server code -->
							<generatorName>spring</generatorName>

							<apiPackage>eu.wiegandt.OpenApiMultiValueParamBugExample.api</apiPackage>
							<modelPackage>eu.wiegandt.OpenApiMultiValueParamBugExample.model</modelPackage>
							<invokerPackage>eu.wiegandt.OpenApiMultiValueParamBugExample.invoker</invokerPackage>
							<packageName>eu.wiegandt.OpenApiMultiValueParamBugExample.server</packageName>

							<!-- pass any necessary config options -->
							<configOptions>
								<serializableModel>true</serializableModel>
								<snapshotVersion>true</snapshotVersion>
								<delegatePattern>true</delegatePattern>
								<useBeanValidation>false</useBeanValidation>
								<performBeanValidation>false</performBeanValidation>
								<dateLibrary>java8</dateLibrary>
								<useTags>false</useTags>
							</configOptions>

							<!-- prevent generation of unnecessary files -->
							<generateApis>true</generateApis>
							<generateModels>true</generateModels>
							<generateApiTests>false</generateApiTests>
							<generateModelTests>false</generateModelTests>
							<generateApiDocumentation>false</generateApiDocumentation>
							<generateModelDocumentation>false</generateModelDocumentation>
							<generateSupportingFiles>true</generateSupportingFiles>
							<supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate>
							<additionalProperties>removeEnumValuePrefix=false</additionalProperties>
						</configuration>
					</execution>
				</executions>
			</plugin>

Steps to reproduce

1. Generate
2. Send multiple parameters with same name

Expected:
All values recieved.

Actual:
Only the first value is recieved.

Example

Here is a GitHub Project with a full working example including a test which shows the described problem:
https://github.com/Nicklas2751/OpenApiMultiValueParamBugExample

Suggest a fix/enhancement

Instead of Map<String,List<String>> uses MultiValueMap.

Start by running the linked OpenApiMultiValueParamBugExample project and its reproduction test, then inspect the Spring generator output produced by the Maven plugin. Confirm that repeated query parameters preserve all values and that the generated parameter type supports them as expected.