Format network policies update time

What does this MR do?

Properly formats updatedAt field in network policies GraphQL endpoint to prevent the following error:

{
  "errors": [
    {
      "message": "Internal server error: undefined method `iso8601' for \"2021-06-08T17:57:10Z\":String"
    }
  ]
}

Follows-up on !63696 (merged).

Screenshots (strongly suggested)

Before	After

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