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{"listlockunspent",
                "\nReturns list of temporarily unspendable outputs.\n"
                "See the lockunspent call to lock and unlock transactions for spending.\n",
                {},
                RPCResult{
                    RPCResult::Type::ARR, "", "",
                    {
                        {RPCResult::Type::OBJ, "", "",
                        {
                            {RPCResult::Type::STR_HEX, "txid", "The transaction id locked"},
                            {RPCResult::Type::NUM, "vout", "The vout value"},
                        }},
                    }
                },
                RPCExamples{
            "\nList the unspent transactions\n"
            + HelpExampleCli("listunspent", "") +