Cardano Updates

Change supported UTXO_BACKEND values to include 'disklmdb' for the existing `V1LMDB` backend in both fast and slow cluster scripts. Add the new `V2LSM` backend option as 'disk'. Update testnet.json files accordingly to reflect these changes.

- Add "empty" as a valid option for UTXO_BACKEND and MIXED_UTXO_BACKENDS
  in both common-start-fast and common-start-slow scripts.
- Implement logic to rotate through mixed UTXO backends for block
  producing nodes, defaulting to "empty" (unset) when specified.
- Update testnet.json documentation to clarify that the default backend
  is "empty" (mem without configuration) if unset.

- Add "empty" as a valid option for UTXO_BACKEND and MIXED_UTXO_BACKENDS
  in both common-start-fast and common-start-slow scripts.
- Implement logic to rotate through mixed UTXO backends for block
  producing nodes, defaulting to "empty" (unset) when specified.
- Update testnet.json documentation to clarify that the default backend
  is "empty" (mem without configuration) if unset.

- Add "legacy" as a valid option for UTXO_BACKEND and MIXED_UTXO_BACKENDS
  in both common-start-fast and common-start-slow scripts.
- Implement logic to rotate through mixed UTXO backends for block
  producing nodes, defaulting to "legacy" (unset) when specified.
- Update testnet.json documentation to clarify that the default backend
  is "legacy" (mem without configuration) if unset.

Change supported UTXO_BACKEND values to include 'disklmdb' for the existing `V1LMDB` backend in both fast and slow cluster scripts. Add the new `V2LSM` backend option as 'disk'. Update testnet.json files accordingly to reflect these changes.