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();
                   ^
{
            RPCHelpMan{"listwalletdir",
                "Returns a list of wallets in the wallet directory.\n",
                {},
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::ARR, "wallets", "",
                        {
                            {RPCResult::Type::OBJ, "", "",
                            {
                                {RPCResult::Type::STR, "name", "The wallet name"},
                            }},
                        }},
                    }
                },
                RPCExamples{
                    HelpExampleCli("listwalletdir", "")
            + HelpExampleRpc("listwalletdir", "")
                },
            }.Check(request);
    UniValue wallets(UniValue::VARR);
    for (const auto& path : ListWalletDir()) {
        UniValue wallet(UniValue::VOBJ);
        wallet.pushKV("name", path.string());
        wallets.push_back(wallet);
    }
    UniValue result(UniValue::VOBJ);
    result.pushKV("wallets", wallets);