Issue #3038991 by bojanz: Prorating logic is not correct for prepaid subscriptions

src/Charge.php

src/Plugin/Commerce/Prorater/FullPrice.php

src/Plugin/Commerce/Prorater/Proportional.php

src/Plugin/Commerce/Prorater/ProraterInterface.php

...Plugin/Commerce/SubscriptionType/SubscriptionTypeBase.php

src/RecurringOrderManager.php

tests/src/Kernel/ChargeTest.php

.../src/Kernel/Plugin/Commerce/Prorater/ProportionalTest.php

tests/src/Kernel/RecurringOrderManagerTest.php