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();
                   ^
HelpExampleCli("listaddressgroupings", "")
            + HelpExampleRpc("listaddressgroupings", "")
                },
            }.Check(request);
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK(pwallet->cs_wallet);
    UniValue jsonGroupings(UniValue::VARR);
    std::map<CTxDestination, CAmount> balances = pwallet->GetAddressBalances();
    for (const std::set<CTxDestination>& grouping : pwallet->GetAddressGroupings()) {
        UniValue jsonGrouping(UniValue::VARR);
        for (const CTxDestination& address : grouping)
        {
            UniValue addressInfo(UniValue::VARR);
            addressInfo.push_back(EncodeDestination(address));
            addressInfo.push_back(ValueFromAmount(balances[address]));
            {
                const auto* address_book_entry = pwallet->FindAddressBookEntry(address);
                if (address_book_entry) {
                    addressInfo.push_back(address_book_entry->GetLabel());
                }
            }
            jsonGrouping.push_back(addressInfo);
        }
        jsonGroupings.push_back(jsonGrouping);
    }
    return jsonGroupings;
}
static UniValue signmessage(const JSONRPCRequest& request)