Cardano Updates

The only remaining use of the `SecurityParam` is for the snapshot policy 🙃

Superseded by the rework of the snapshot policy for predictable snapshots, with
dedicated new tests

It is no longer needed by the predictable snapshotting logic.

Also make sure to account for the fact that the DbChangelog might have gotten
pruned between opening and committing the forker.

regarding the previous few commits

It was already superseded in the most important places due to
`LedgerDbPruneBeforeSlot`. Its remaining use cases are non-essential:

 - Replay on startup.

   In this case, we never roll back, so not maintaining k states is actually an
   optimization here. We can also remove the now-redundant `InitDB.pruneDb`
   function.

 - Internal functions used for db-analyser.

   Here, we can just as well use `LedgerDbPruneAll` (which is used by
   `pruneToImmTipOnly`) as we never need to roll back.

 - Testing.

   In particular, we remove some DbChangelog tests that previously ensured that
   only at most @k@ states are kept. This is now no longer true; that property
   is instead enforced by the LedgerDB built on top of the DbChangelog.

   A follow-up commit in this PR enriches the LedgerDB state machine test to
   make sure that the public API functions behave appropriately, ensuring that
   we don't lose test coverage (and also testing V2, which previously didn't
   have any such tests).

Make sure that we correctly fail when trying to roll back too far.

Also remove the version for `ValidatedChainDiff` as it is unused.