Issue #3139270 by frankdesign, cYu: Warning cleanup and restore of "choose by default" feature

On fresh install of D9, this takes care of the install warning mentioned in #3017841, the "Notice: Only variables should be assigned by reference" mentioned at drupal.org/project/commerce_choose_price/issues/3139270#comment-14094739 and restores the pre-D9 readiness feature of allowing choose your own price to be the default display since I didn't see any note on why it was removed and it is useful to myself and others.