Just reference the documented terraform solution using jsonencode:

resource "kubernetes_secret" "example" {
  metadata {
    name = "docker-cfg"
  }

  type = "kubernetes.io/dockerconfigjson"

  data = {
    ".dockerconfigjson" = jsonencode({
      auths = {
        "${var.registry_server}" = {
          "username" = var.registry_username
          "password" = var.registry_password
          "email"    = var.registry_email
          "auth"     = base64encode("${var.registry_username}:${var.registry_password}")
        }
      }
    })
  }
}

which is equivalent to:

$ kubectl create secret docker-registry docker-cfg --docker-server=${registry_server} --docker-username=${registry_username} --docker-password=${registry_password} --docker-email=${registry_email}