Simplify CardanoHardForkTriggers

[amesgen]

Closes #1281

Instead of letting the user provide several TriggerHardForks, only let them provide CardanoHardForkTriggers, a restricted version that should make protocolInfoCardano more straightforward and less error-prone.

data CardanoHardForkTrigger blk =
    -- | Trigger the hard fork when the ledger protocol version is updated to
    -- the default for that era (@'L.eraProtVerLow' \@('ShelleyBlockLedgerEra'
    -- blk)@). Also see 'TriggerHardForkAtVersion'.
    CardanoTriggerHardForkAtDefaultVersion
  |
    -- | Trigger the hard fork at the given epoch. For testing only. Also see
    -- 'TriggerHardForkAtEpoch'.
    CardanoTriggerHardForkAtEpoch EpochNo

It is (intentionally) no longer possible to directly (though still manually, also see the changelog entry) to use a non-default version trigger. However, this feature was used in the Cardano ThreadNet test (as Byron had an intra-era HF), which we resolve by modifying the initial Byron protocol version (see the corresponding Haddocks).

In the node, this will result in the removal of the (unused) TestXxxHardForkAtVersion config fields.