fix(DatePicker): fix reference for onOpen

[guidari]

Closes #13793

Fixed the reference in the onOpen prop. Before it was creating a new function that was returning the onChange function, and now the onChange is saving the reference without creating a new function.

Testing / Reviewing

Go to DatePicker.stories.js and add a console.log

Example:

export const SingleWithCalendar = () => (
  <DatePicker datePickerType="single" onOpen={() => console.log('onOpen')}>
    <DatePickerInput
      placeholder="mm/dd/yyyy"
      labelText="Date Picker label"
      id="date-picker-single"
      size="md"
    />
  </DatePicker>
);

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