flutter_paystack fails to compile on Flutter 3: pin the fixed git commit

If flutter_paystack breaks your Android build right after a Flutter 3 upgrade, do not downgrade Flutter: the published 1.0.5+1 plugin predates the embedding changes. Point the dependency at the upstream git repo pinned to the fixed commit in pubspec.yaml. Whenever a Paystack Flutter plugin stops compiling after an SDK upgrade, check the upstream repo for a fixed commit before touching your own code.

Context: Stack Overflow #72461079 (accepted answer): After upgrading to Flutter 3, the published flutter_paystack 1.0.5+1 plugin fails to compile on Android with Kotlin type mismatches (Activity? vs Activity, BinaryMessenger? vs BinaryMessenger). The accepted fix is to stop depending on the published version and pin the plugin to a git ref of the upstream repo that contains the Flutter 3 compatibility fix.