v5.1.0

This updates reverts a few breaking changes made in v5.0.0 in order to keep things simple.

• The onDetect method has been reinstated in the MobileScanner widget, but is nullable. You can
  still listen to MobileScannerController.barcodes directly by passing null to this parameter.
• The autoStart attribute has been reinstated in the MobileScannerController and defaults to true. However, if you want
  to control which camera is used on start, or you want to manage the lifecycle yourself, you should set
  autoStart to false and manually call MobileScannerController.start({CameraFacing? cameraDirection}).
• The controller is no longer required in the MobileScanner widget. However if provided, the user should take care
  of disposing it.
• [Android] Revert Gradle 8 back to Gradle 7, to be inline with most Flutter plugins and prevent build issues.
• [Android] Revert Kotlin back from 1.9 to 1.7 to be inline with most Flutter plugins. Special 1.9 functionality
  has been refactored to be compatible with 1.7.