Currently we store PP proofs encoded but without their selector. It means we don't remember which PP version generated which proof.

We probably should at least store the pp vkey selector alongside the proof in our storage, eg. so that we could send to l1 PPs generated with older PP versions than the one in use at the current certificate time.

We will probably want to do this as part of all our storage refactorings, so that we have a single migration to deal with.