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();
                   ^
}
    return balances;
}
static UniValue getwalletinfo(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    const CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    RPCHelpMan{"getwalletinfo",
                "Returns an object containing various wallet state info.\n",
                {},
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {
                        {RPCResult::Type::STR, "walletname", "the wallet name"},
                        {RPCResult::Type::NUM, "walletversion", "the wallet version"},
                        {RPCResult::Type::STR_AMOUNT, "balance", "DEPRECATED. Identical to getbalances().mine.trusted"},
                        {RPCResult::Type::STR_AMOUNT, "unconfirmed_balance", "DEPRECATED. Identical to getbalances().mine.untrusted_pending"},
                        {RPCResult::Type::STR_AMOUNT, "immature_balance", "DEPRECATED. Identical to getbalances().mine.immature"},
                        {RPCResult::Type::NUM, "txcount", "the total number of transactions in the wallet"},
                        {RPCResult::Type::NUM_TIME, "keypoololdest", "the " + UNIX_EPOCH_TIME + " of the oldest pre-generated key in the key pool. Legacy wallets only."},
                        {RPCResult::Type::NUM, "keypoolsize", "how many new keys are pre-generated (only counts external keys)"},
                        {RPCResult::Type::NUM, "keypoolsize_hd_internal", "how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)"},
                        {RPCResult::Type::NUM_TIME, "unlocked_until", "the " + UNIX_EPOCH_TIME + " until which the wallet is unlocked for transfers, or 0 if the wallet is locked"},
                        {RPCResult::Type::STR_AMOUNT, "paytxfee", "the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB"},
                        {RPCResult::Type::STR_HEX, "hdseedid",  true, "the Hash160 of the HD seed (only present when HD is enabled)"},
                        {RPCResult::Type::BOOL, "private_keys_enabled", "false if privatekeys are disabled for this wallet (enforced watch-only wallet)"},