'fun onBackPressed(): Unit' is deprecated

Android 140 nightlies fail:

e: warnings found and -Werror specified                                                                                                       
w: file:///var/tmp/build/geckoview-759ac06a5ff7/mobile/android/fenix/app/src/main/java/org/mozilla/fenix/tor/TorSecurityLevelFragment.kt:113:31 'fun onBackPressed(): Unit' is deprecated. This method has been deprecated in favor of using the
      {@link OnBackPressedDispatcher} via {@link #getOnBackPressedDispatcher()}.                                                              
      The OnBackPressedDispatcher controls how back button events are dispatched                                                              
      to one or more {@link OnBackPressedCallback} objects.