Incorrect schema definition for "preload"

Done, please review! We should also cherry-pick this in 4.x!

This is how "preload" looked before:

I'm wondering why config inspector did not complain... but perhaps it wasn't possible to compare due to the specific case...

Did you test the update hook? That's the dangerous part...

Forgot to clear old keys.

Ok, works as expected now! This is the new output after running the update hook:

\Drupal::configFactory()->getEditable(...)->get(...) array (2)
⇄amountBefore => integer 1
⇄amountAfter => integer 1

Actually, since we are using the js spread operator in 5.x we should check how the spread operator handles the array first, before commiting this.

Yep, thought so... preload is now a js object instead of an array, probably breaking its functionality.

Well, I guess, we just close it as "won't fix" then.

@Grevil: I'll reopen this, so we don't forget to talk about this. I don't get it here... ;)
We can of cours decide to close it then.

Finally:
The schema is correct as - is, without any change. It just allows unlimited entries, while only two are allowed.

The proposed MR would make it an object, which would be even worse.
The alternative fix would have been to explicitly save the two values separately and turn them into an array in PHP or JS, but it should be OK to just keep it this way.