Add breaking changes documentation for `allow_superuser` by default

As part of `9.0`, the default value of `allow_superuser` is set to `false`, instead of `true`. 

This means that a user running Logstash as a superuser, will see the Logstash process fail to start upon upgrade to `9.0`.

While we strongly recommend keeping this option, it is possible to revert to the previous behaviour by setting this flag to `true`