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{"upgradewallet",
        "\nUpgrade the wallet. Upgrades to the latest version if no version number is specified\n"
        "New keys may be generated and a new wallet backup will need to be made.",
        {
            {"version", RPCArg::Type::NUM,  strprintf("%d", FEATURE_LATEST), "The version number to upgrade to. Default is the latest wallet version"}
        },
        RPCResults{},
        RPCExamples{
            HelpExampleCli("upgradewallet", "169900")
            + HelpExampleRpc("upgradewallet", "169900")
        }
    }.Check(request);
    RPCTypeCheck(request.params, {UniValue::VNUM}, true);
    EnsureWalletIsUnlocked(pwallet);
    int version = 0;