This change introduces a breaking change to miner.etherbase is configured.

Previously, users did not need to explicitly set the  etherbase address via flag, since 'first' local account was used as etherbase automatically. This change removes the  "default first account" feature.

In Proof-of-stake world, the fee recipient address is provided by CL, and not configured in Geth any more - meaning that miner.etherbase is mostly for legacy networks(pow, clique networks etc).