[BUG][Java] Raw MultiValueMap instances created for formParams

[jonfreedman]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Compiler warnings are generated from Api classes as formParams is not parameterised.

openapi-generator version

openapi-generator-maven-plugin 4.3.0 - needs to be fixed in master

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/api.mustache#L119

Command line used for generation

n/a - this is present in your samples: https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/api/StoreApi.java#L88

Steps to reproduce

n/a - this is present in your samples: https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/java/resttemplate/src/main/java/org/openapitools/client/api/StoreApi.java#L88

Related issues/PRs

n/a

Suggest a fix

Just add the parameters: <String, Object>