Call toMinorUnits from the service

Problem / Motivation

In earlier versions, the module was calling $this->toMinorUnits(), which worked because the method was defined in PaymentGatewayBase.
With the update to Drupal Commerce 3.1, this method was removed from PaymentGatewayBase, so it is no longer available directly.
We now need to use the appropriate service to handle this conversion.

Proposed Resolution

Update the implementation to call the method from the minorUnitsConverter service instead of PaymentGatewayBase.