Revisit `base_fee_per_gas` optional field

[avilagaston9]

We set base_fee_per_gas header field as optional solely to run the eth capability suite of tests of the devp2p Hive simulator (#394). This adds pre-merge logic and we want to support only post-merge networks. We should revisit this change in the future and think if we can revert these changes.

[mpaulucci]

One solution could be to update the Hive simulation to provide a post-merge genesis file, see: https://github.com/ethereum/hive/tree/master/simulators/devp2p

Edit: It seems it is already post merge: ethereum/go-ethereum#28340