Update Java compatibility version above 8 to avoid obsolescence warnings

[Zalezny]

While building the Flutter APK, the compiler throws warnings regarding the Java source and target versions used in this package. Java 8 is now considered obsolete and will be removed in future releases of the Android Gradle Plugin.

warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
Note: .../disable_battery_optimization-1.1.2/android/src/main/java/in/jvapps/disable_battery_optimization/utils/PrefUtils.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

Please consider updating compileOptions and kotlinOptions in the Android build.gradle file to target Java 11 (or higher).