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();
                   ^
} else {
            labels.push_back(address_book_entry->GetLabel());
        }
    }
    ret.pushKV("labels", std::move(labels));
    return ret;
}
static UniValue getaddressesbylabel(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    const CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
            RPCHelpMan{"getaddressesbylabel",
                "\nReturns the list of addresses assigned the specified label.\n",
                {
                    {"label", RPCArg::Type::STR, RPCArg::Optional::NO, "The label."},
                },
                RPCResult{
                    RPCResult::Type::OBJ_DYN, "", "json object with addresses as keys",
                    {
                        {RPCResult::Type::OBJ, "address", "json object with information about address",
                        {
                            {RPCResult::Type::STR, "purpose", "Purpose of address (\"send\" for sending address, \"receive\" for receiving address)"},
                        }},
                    }
                },
                RPCExamples{
                    HelpExampleCli("getaddressesbylabel", "\"tabby\"")