PMPro_Field::displayAtCheckout is deprecated in v3.4 +

[kimcoleman]

Describe the bug
PMPro_Field::displayAtCheckout is deprecated since version 3.4. The replacement for this method is to use PMPro_Field_Group::display() instead.

To Reproduce
Steps to reproduce the behavior:

1. Navigate to the frontend testimonial display form.
2. If WP_DEBUG is enabled and displayed you will see the deprecation warning.
3. See error

Expected behavior
No warnings should be shown. We should support the current method as a fallback if the site is not running a PMPro version with the new PMPro_Field_Group::display method.

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and confirmed this bug occurs when only Paid Memberships Pro plugin is active.
• This bug happens with a default WordPress theme active, or Memberlite.
• I can reproduce this bug consistently using the steps above.