Check if `next_cert_issuable_on` is used
There is a TODO in the code: https://git.duniter.org/nodes/rust/duniter-v2s/-/blob/56998122e42afd2c2c1642a72a6772a82490ccda/node/src/chain_spec/gen_genesis_data.rs#L192
/// timestamp at which the next cert can be emitted
next_cert_issuable_on: TimestampV1, // TODO: unused?
We should check this value is correctly used.