fix: IAP modal z-index (#108)

Author: FlorianKe

src/Resources/app/administration/src/module/sw-in-app-purchases/component/sw-in-app-purchase-checkout/sw-in-app-purchase-checkout.scss

@@ -1,4 +1,6 @@
 .sw-in-app-purchase-checkout {
+    z-index: 1050;
+
     .sw-modal {
         &__footer {
             grid-template-columns: 1fr;