* Apply [Boy scout rule](https://github.com/akka/akka/blob/master/CONTRIBUTING.md#additional-guidelines)
* [Bug#23329] PersistentFSM: andThen callbacks are not executed when stay()
* [fixes#23329] PersistentFSM: andThen callbacks go execute when stay()
* At documentation: there is nothing said about events applied should decide to invoke andThen or not.
* At code: andThen callback can be specified whether any events/state transition applied or not.
Added call to nextState.afterTransitionDo(stateData) even if there are no eventsToPersist
`State.using` is a valid pattern in regular FSM. However, for persistent FSM
it makes no sense and is marked as `private[akka]`. The `private[akka]` modifier does
not translate into bytecode so that Java users won't see that it should not be used.
Deprecating it will allow us to remove it in the future.
Fixes#23739.
* backport of the timers from Akka Typed, #16742
* also fixed a small bug in FSM timers, which could result in that
a timer from a previous incarnation was let through to new
incarnation after restart
* no more need for the complicated "how to" section in docs of
how to schedule periodic messages
* Use snapshot-after in config to periodical snapshot in PersistentFSM (#21563)
* akka.persistence.fsm.snapshot-after is either off or a numerical value
* Use Akka Extension for snapshot-after in PersistentFSM
* marked PersistentFSM as experimental
* also changed the order of some sections in migration guide
* link to 2.2->2.3 migration guide
* update experimental index page
