Remove `TriggerHardForkAtEpoch`

Due to #395, we have good evidence that no use case is **fundamentally**/**crucially** relying on `TriggerHardForkAtEpoch`. Hence, we would like to remove them to simplify the possible trajectories of the HFC (also see [this HWW section](https://ouroboros-consensus.cardano.intersectmbo.org/docs/for-developers/HardWonWisdom#the-safe-zone-semantics)).

Goals of this ticket:

- Remove the `TriggerHardForkAtEpoch` constructor, and adapt documentation accordingly.
- Introduce a different way for users to specify that they wish to directly skip to a later era. (The current interface/configuration options could be kept as is for a migration period.)
- Provide guidance to migrate away from the remaining use cases of `TriggerHardForkAtEpoch` mentioned in #395.