[BUG][KOTLIN] Wrong import for fields with "format: date"

Created by: toXel

Description

Models that have a field with type: string and format: date are generated with a wrong import when using threetenbp as the date library.

openapi-generator version

3.3.4

OpenAPI declaration file content or url

Every OpenAPI spec with at least one field like this will trigger the bug:

created:
    type: string
    format: date

Command line used for generation

Gradle plugin was used with the following config:

openApiGenerate {
    generatorName = "kotlin"
    inputSpec = "$rootDir/openApiClient/OpenAPI.yml".toString()
    outputDir = "$rootDir/openApiClient".toString()
    configOptions = [
            dateLibrary   : "threetenbp",
            collectionType: "list"
    ]
}

Steps to reproduce

1. Use OpenAPI spec with at least one model with a format: date field
2. Generate the client
3. There will be Unresolved reference: org compiler errors in the model files