Expire forge state of past eras

In input-output-hk/ouroboros-network#2753, the hard fork combinator combines the `BlockForging` of all eras into a single one, using the `BlockForging` of the current era.

When transitioning from one era to the next, the forge state of the previous era will be kept around while it is not needed anymore. If we know the transition is stable (`k` old), we could *expire* it. For example, we could securely forget the KES key in question.