statement-distribution: prep for re-enabling (#4431)

In preparation for launching re-enabling
(https://github.com/paritytech/polkadot-sdk/issues/2418), we need to
adjust the disabling strategy of statement-distribution to use the relay
parent's state instead of the latest state (union of active leaves).
This will also ensure no raciness of getting the latest state vs
accepting statements from disabling validators at the cost of being more
lenient/potentially accepting more statements from disabled validators.

- [x] PRDoc