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();
                   ^
);
    CAmount fee;
    int change_position;
    bool rbf = pwallet->m_signal_rbf;
    const UniValue &replaceable_arg = request.params[3]["replaceable"];
    if (!replaceable_arg.isNull()) {
        RPCTypeCheckArgument(replaceable_arg, UniValue::VBOOL);
        rbf = replaceable_arg.isTrue();
    }
    CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], rbf);
    FundTransaction(pwallet, rawTx, fee, change_position, request.params[3]);
    
    PartiallySignedTransaction psbtx(rawTx);
    
    bool bip32derivs = request.params[4].isNull() ? true : request.params[4].get_bool();
    bool complete = true;
    const TransactionError err = pwallet->FillPSBT(psbtx, complete, 1, false, bip32derivs);
    if (err != TransactionError::OK) {
        throw JSONRPCTransactionError(err);
    }
    
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << psbtx;
    UniValue result(UniValue::VOBJ);
    result.pushKV("psbt", EncodeBase64(ssTx.str()));
    result.pushKV("fee", ValueFromAmount(fee));
    result.pushKV("changepos", change_position);
    return result;
}
static UniValue upgradewallet(const JSONRPCRequest& request)