Remove `StateTransition` schedule in favor of using observers

## What problem does this solve or what need does it fill?

Updating the app state is a very flexible and expressive operation, which is fundamentally deferred. States typically changes only once per frame, after `PostUpdate`.

This can lead to unwanted delays when handling multiple dependent transitions at once.

## What solution would you like?

As discussed in #15127, it would be nice to remove this meta-schedule, and instead run the various `OnEnter` / `OnExit` / `OnTransition` schedules on demand, leveraging the new observers. This is a good fit for "highly complex rarely exercised logic", and resolves the delay problem mentioned above while simplifying the mental model for consumers.

#15127 should be tackled in concert with this, removing `NextState` completely, and relying entirely on commands to handle transitions.

## What alternative(s) have you considered?

Users can manually add more state transition systems to their schedule, including in their own schedules that run inside of `Main`. This is somewhat messy, nonstandard and hard to discover.

## Additional context

Discussed briefly on Discord with @MiniaczQ [here](https://discordapp.com/channels/691052431525675048/749335865876021248/1282862231570092033).

This design was not originally taken (including in the substates refactor) because observers didn't exist at the time!