build!: Downgrade java version to java 8

[IsaacGuiotto]

Fixed the following issue:

flutterEngine.getPlugins().add(new com.mews.kiosk_mode.KioskModePlugin());
                                                            ^
  bad class file: /Users/${path-to-project}/build/kiosk_mode/intermediates/compile_library_classes_jar/debug/bundleLibCompileToJarDebug/classes.jar(/com/mews/kiosk_mode/KioskModePlugin.class)
    class file has wrong version 65.0, should be 61.0
    Please remove or make sure it appears in the correct subdirectory of the classpath.
1 error