Use display name instead of label on coupon redemption form

Describe your bug or feature request.

The coupon display name should be used instead of coupon label on CouponRedemption.php.

The commerce-coupon-redemption-form.html.twig is capable to use the "display_name" variable thanks to CouponRedemption.php, but instead of passing the display name, the label is passed.

There is also a "// @todo Use the promotion display name once added." comment on CouponRedemption.php.

Well, now the display name has been added since one or two years, so we can safely update this code either.

I'm attaching a working patch.

Thank you.