Remove traces of the `StateNodeConfig["key"]`

[Andarist]

Follow-up to https://github.com/statelyai/xstate/pull/3455/files#r967087849

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[davidkpiano]

Before merging this in, let me think about naming...

• id here should serve as only the root state node ID
• The id (or name?) that the parent gives the child is for identifying the entire child actor
• The root state node ID should not be used to self-identify the actor; only the state node

With that said, is id still the best name? I think probably yes, but we should document the difference in behavior from v4 that this:

spawn(someMachine, 'some-id');

Is different from this:

const someMachine = createMachine({
  id: 'rootNodeId'
})