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);
    const CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
            RPCHelpMan{"listtransactions",
                "\nIf a label name is provided, this will return only incoming transactions paying to addresses with the specified label.\n"
                "\nReturns up to 'count' most recent transactions skipping the first 'from' transactions.\n",
                {
                    {"label|dummy", RPCArg::Type::STR, RPCArg::Optional::OMITTED_NAMED_ARG, "If set, should be a valid label name to return only incoming transactions\n"
                          "with the specified label, or \"*\" to disable filtering and return all transactions."},
                    {"count", RPCArg::Type::NUM,  "10", "The number of transactions to return"},
                    {"skip", RPCArg::Type::NUM,  "0", "The number of transactions to skip"},
                    {"include_watchonly", RPCArg::Type::BOOL,  "true for watch-only wallets, otherwise false", "Include transactions to watch-only addresses (see 'importaddress')"},
                },
                RPCResult{
                    RPCResult::Type::ARR, "", "",
                    {
                        {RPCResult::Type::OBJ, "", "", Cat(Cat<std::vector<RPCResult>>(
                        {
                            {RPCResult::Type::BOOL, "involvesWatchonly", "Only returns true if imported addresses were involved in transaction."},