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();
                   ^
{
                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The txid to be bumped"},
                    {"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "",
                        {
                            {"confTarget", RPCArg::Type::NUM,  "wallet default", "Confirmation target (in blocks)"},
                            {"fee_rate", RPCArg::Type::NUM,  "fall back to 'confTarget'", "fee rate (NOT total fee) to pay, in " + CURRENCY_UNIT + " per kB\n"
            "                         Specify a fee rate instead of relying on the built-in fee estimator.\n"
                                     "Must be at least 0.0001 " + CURRENCY_UNIT + " per kB higher than the current transaction fee rate.\n"},
                            {"replaceable", RPCArg::Type::BOOL,  "true", "Whether the new transaction should still be\n"
            "                         marked bip-125 replaceable. If true, the sequence numbers in the transaction will\n"
            "                         be left unchanged from the original. If false, any input sequence numbers in the\n"
            "                         original transaction that were less than 0xfffffffe will be increased to 0xfffffffe\n"
            "                         so the new transaction will not be explicitly bip-125 replaceable (though it may\n"
            "                         still be replaceable in practice, for example if it has unconfirmed ancestors which\n"
            "                         are replaceable)."},
                            {"estimate_mode", RPCArg::Type::STR,  "UNSET", "The fee estimate mode, must be one of:\n"
            "         \"UNSET\"\n"
            "         \"ECONOMICAL\"\n"
            "         \"CONSERVATIVE\""},
                        },
                        "options"},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "", {
                        {RPCResult::Type::STR, "psbt", "The base64-encoded unsigned PSBT of the new transaction. Only returned when wallet private keys are disabled."},
                        {RPCResult::Type::STR_HEX, "txid", "The id of the new transaction. Only returned when wallet private keys are enabled."},
                        {RPCResult::Type::STR_AMOUNT, "origfee", "The fee of the replaced transaction."},
                        {RPCResult::Type::STR_AMOUNT, "fee", "The fee of the new transaction."},
                        {RPCResult::Type::ARR, "errors", "Errors encountered during processing (may be empty).",
                        {