Chain information: load the storage value of the GrandPa set ID when the API is too old

[tomaka]

If the GrandPa API is too old, we currently generate an error:

smoldot/lib/src/chain/chain_information/build.rs

Line 564 in 0ffbefc

result: Err(Error::GrandpaApiTooOld),

However, paritytech/substrate#3472 introduced a current_set_id storage item. This PR is old enough that it is present in the genesis block of every chain in existence. Rather than return an error, we could load this storage item.

This would make it possible to warp sync to early Kusama blocks, which isn't very useful, but also to simplify the full node database implementation by reviving paritytech/smoldot#2909.

[tomaka]

Actually this is pointless, as we need paritytech/substrate#7829 in order to know the Babe next epoch information.
Before that PR, the information about the next epoch isn't stored in the runtime at all, so there's no other option but to put it into the database.