The wallet code uses `Optional::emplace()`, which was only added in boost 1.56, see https://github.com/boostorg/optional/commit/2e583aaf30964afd66c1284aa71d0d8b42fa440d

So for 0.21.0 there are two options:
* Work around `emplace` and keep the minimum boost version below 1.56
* Require boost 1.56


```
wallet/rpcwallet.cpp: In function ‘UniValue listsinceblock(const JSONRPCRequest&)’:
wallet/rpcwallet.cpp:1560:16: error: ‘Optional<int>’ has no member named ‘emplace’
         height.emplace();
                ^
wallet/rpcwallet.cpp:1561:19: error: ‘Optional<int>’ has no member named ‘emplace’
         altheight.emplace();
                   ^
throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, errors[0].original);
                break;
            case feebumper::Result::INVALID_REQUEST:
                throw JSONRPCError(RPC_INVALID_REQUEST, errors[0].original);
                break;
            case feebumper::Result::INVALID_PARAMETER:
                throw JSONRPCError(RPC_INVALID_PARAMETER, errors[0].original);
                break;
            case feebumper::Result::WALLET_ERROR:
                throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
                break;
            default:
                throw JSONRPCError(RPC_MISC_ERROR, errors[0].original);
                break;
        }
    }
    UniValue result(UniValue::VOBJ);
    
    
    if (!pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        if (!feebumper::SignTransaction(*pwallet, mtx)) {
            throw JSONRPCError(RPC_WALLET_ERROR, "Can't sign transaction.");
        }
        uint256 txid;
        if (feebumper::CommitTransaction(*pwallet, hash, std::move(mtx), errors, txid) != feebumper::Result::OK) {
            throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
        }
        result.pushKV("txid", txid.GetHex());
    } else {
        PartiallySignedTransaction psbtx(mtx);