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);
    const CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
            RPCHelpMan{"listaddressgroupings",
                "\nLists groups of addresses which have had their common ownership\n"
                "made public by common use as inputs or as the resulting change\n"
                "in past transactions\n",
                {},
                RPCResult{
                    RPCResult::Type::ARR, "", "",
                    {
                        {RPCResult::Type::ARR, "", "",
                        {
                            {RPCResult::Type::ARR, "", "",
                            {
                                {RPCResult::Type::STR, "address", "The bitcoin address"},
                                {RPCResult::Type::STR_AMOUNT, "amount", "The amount in " + CURRENCY_UNIT},
                                {RPCResult::Type::STR, "label",  true, "The label"},
                            }},
                        }},
                    }
                },
                RPCExamples{