Figure out whether `OnTimestampSet = Aura` affect us

[Dengjianping]

It happened to cumulus release-v0.9.320.
Related pr: paritytech/cumulus#1877

@Garandor

[Garandor]

I suppose a description in that linked PR would've been helpful, but anyways,

It's used for a sanity check by Aura in https://github.com/paritytech/substrate/blob/polkadot-v0.9.26/frame/aura/src/lib.rs#L299

Closing as WONTFIX as it does not affect us:

1. We no longer use Aura slots
2. Nimbus slots are defined by RelaychainBlockNumberProvider instead of cumulus_pallet_parachain_system::pallet_timestamp so they don't depend on RTC time

Thanks for raising this @Dengjianping