[bug][Kotlin] Default typeMapping for 'date' should be LocalDate

Created by: legzo

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?

Description

When a spec specifies a property as :

"type": "string",
"format": "date",

the Kotlin generator generates the corresponding field as a LocalDateTime and later, the parsing fails when Jackson is used (Gson too) saying it cannot find a converter for this type.

As I understand the spec, "format":"date" should result in a LocalDate and "format":"date-time" in a LocalDateTime

openapi-generator version

I tried version 3.3.4, but I read the code of master and the problem is still present.

Steps to reproduce

Use as an input any spec that has a property configured as :

"type": "string",
"format": "date",

Suggest a fix/enhancement

I think there is only one line to change at https://github.com/OpenAPITools/openapi-generator/blob/0b66fa5c8214e2907a326fa8684cb2adcfa438a6/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractKotlinCodegen.java#L175

it should be

 typeMapping.put("Date", "java.time.LocalDate");

I can provide a PR if you think it is really a bug.

Cheers.