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();
                   ^
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    RPCHelpMan{"sendmany",
                "\nSend multiple times. Amounts are double-precision floating point numbers." +
        HELP_REQUIRING_PASSPHRASE,
                {
                    {"dummy", RPCArg::Type::STR, RPCArg::Optional::NO, "Must be set to \"\" for backwards compatibility.", "\"\""},
                    {"amounts", RPCArg::Type::OBJ, RPCArg::Optional::NO, "The addresses and amounts",
                        {
                            {"address", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "The bitcoin address is the key, the numeric amount (can be string) in " + CURRENCY_UNIT + " is the value"},
                        },
                    },
                    {"minconf", RPCArg::Type::NUM, RPCArg::Optional::OMITTED_NAMED_ARG, "Ignored dummy value"},
                    {"comment", RPCArg::Type::STR, RPCArg::Optional::OMITTED_NAMED_ARG, "A comment"},
                    {"subtractfeefrom", RPCArg::Type::ARR, RPCArg::Optional::OMITTED_NAMED_ARG, "The addresses.\n"
            "                           The fee will be equally deducted from the amount of each selected address.\n"
            "                           Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
            "                           If no addresses are specified here, the sender pays the fee.",
                        {
                            {"address", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Subtract fee from this address"},