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{"abandontransaction",
                "\nMark in-wallet transaction <txid> as abandoned\n"
                "This will mark this transaction and all its in-wallet descendants as abandoned which will allow\n"
                "for their inputs to be respent.  It can be used to replace \"stuck\" or evicted transactions.\n"
                "It only works on transactions which are not included in a block and are not currently in the mempool.\n"
                "It has no effect on transactions which are already abandoned.\n",
                {
                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                },
                RPCResult{RPCResult::Type::NONE, "", ""},
                RPCExamples{
                    HelpExampleCli("abandontransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
            + HelpExampleRpc("abandontransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
                },
            }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    uint256 hash(ParseHashV(request.params[0], "txid"));
    if (!pwallet->mapWallet.count(hash)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid or non-wallet transaction id");
    }
    if (!pwallet->AbandonTransaction(hash)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not eligible for abandonment");
    }
    return NullUniValue;
}
static UniValue backupwallet(const JSONRPCRequest& request)