cl/beacon: fee recipient ignores Gloas proposer preferences

Block production resolves the fee recipient only from the `prepare_beacon_proposer` registrations:

```go
feeRecipient, registered := a.validatorParams.GetFeeRecipient(proposerIndex)
```

Under Gloas a signed `ProposerPreferences` message can carry one as well. The same closure already reads that message for `TargetGasLimit`, guarded by `pref.Message.ValidatorIndex == proposerIndex`, so the value is in scope at the point the fee recipient is chosen.

Two things follow, in increasing order of scope:

- the warning about a missing registration is inaccurate when a matching preference exists, since one is in fact available;
- more importantly, a proposer that registered only through preferences builds with the zero address and gives that block's fees away.

Deciding the precedence between the two sources is the actual question here, which is why it is not folded into #23274.

Reported by @yperbasis and @domiwei while reviewing #23274.