Type Date must be string according to the OpenAPI specification

[vadim-mve]

Underlying type of Date is null.Time and this causes problems with go-oas3 which generates invalid code like this

func (body Payer) Validate() error {
	return validation.ValidateStruct(&body,
		validation.Field(&body.BirthDate, validation.Skip.When(body.BirthDate == ""), validation.RuneLength(10, 10)))
}

validation.Skip tries to compare null.Time to ""

Invalid code generated here:
https://github.com/mikekonan/go-oas3/blob/main/generator/generator.go#L580

According to specification underlining type of Date must be string