In 2.6, you could access the amount via $coupon->coupon_amount. Or legacy code incorrectly handles $coupon->amount instead. 7778583340/includes/class-wc-coupon.php (L102)
This PR handles both since the RCs and betas allowed `->amount` and I don't want to break anything that may be accessing it that way..
To Test:
* `phpunit --filter=test_coupon_backwards_compat_props_use_correct_getters`