[BUG] [kotlin-spring] Missing validation on collection of models

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

We have hand-coded our models, and are now moving over to schema-first so that we can both be certain that our schemas are correct, and ease development by not having to implement everything ourselves.

However, one of our validation requirements don't seem to be included when using the kotlin-spring generator, while it is with the normal java spring generator.
Specifically the validation we're missing is that the EmailAddresses used in an Email field is valid.
In other words, we can validate that the to: List<EmailAddress> field has at least one entry, but we can't validate that to[0].email actually is validated with minlength: 5. We've also used a regex pattern on the EmailAddress.email field with no success.

openapi-generator version

Version 4.2.2. I don't think it's a regression, but I haven't tested any previous versions.

OpenAPI declaration file content or url

https://gist.github.com/tofagerl/f9c315662e2ae2d22b5a75ba803c1171

Command line used for generation

Using the gradle plugin:

task generateEmailSpec(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask) {
    validateSpec = true
    inputSpec = "src/main/openapi/email.json"
    generatorName = "kotlin-spring"
    outputDir = "$generatedSources/email"
    apiPackage = "org.entur.email.api"
    modelPackage = "org.entur.email.models"
    configOptions = [
            enumPropertyNaming: 'UPPERCASE',
            interfaceOnly     : 'true',
            gradleBuildFile   : 'false',
            title             : 'Email',
            useBeanValidation: 'true',
            performBeanValidation: 'true'
    ]
}

Steps to reproduce

Run task generateEmailSpec to generate the models, see that there is no @Valid annotation on the field Email.to
Change generatorName to 'spring', and run the task again. Note that the field is now marked as @Valid.

Related issues/PRs

I couldn't find anything

Suggest a fix

I haven't looked much at the codebase, so I don't know how, but it seems like adding the annotation should be possible.

Expected output

        @get:NotNull @get:Size(min=1) 
		@Valid
        @JsonProperty("to") val to: kotlin.collections.List<EmailAddress>,

Actual output

        @get:NotNull @get:Size(min=1) 
        @JsonProperty("to") val to: kotlin.collections.List<EmailAddress>,

Start with the Gradle generateEmailSpec task in the issue and reproduce the kotlin-spring output using the linked OpenAPI declaration. Compare the generated Email.to model field with the spring generator output, then verify that the collection carries @Valid so nested EmailAddress.email constraints are applied.